Basic server-side mod debugging howto

Hi All.  Let me first say that yes, I am a total noob.  I'm setting up a Java edition, with Forge, server with mods for my kiddo, and I have plenty of sysadmin experience but very little Minecraft knowledge.  I'm trying to determine whether or not my server is loaded the mods / modpack that I want it to be, and if not, what's going wrong.  There's no crash, no stack trace, no debugging log files, it's just not working as expected.  The reported behavior is this:

• The client-side game appears to work fine and be loading the desired mods; single-player works as expected.
• The server (Java edition 1.12.2;  forge 14.23.5.2854; modpack "Avatar the Last Minebender", manifest.json below; running on linux - Debian 10 (buster)) appears to have all the right mods installed, and the modded client is able to connect with the server w/o error)
• When the player (using the client, naturally) attempts to place a mod-specific block, it seems to appear for a brief moment and then vanish.  When he places a vanilla block, it works as expected.

I'll take any kind of help I can get, but what I'm specifically looking for is this:  Is there a way to get a debug/trace log that will show the mod discovery & loading process, so I can confirm that the mods I think are being loaded really are?  And similarly, can I enable logging that would capture what's going on when (seemingly) the server "rejects" the mod-specific block placed by the client?  That seems like the kind of significant event that would produce a warning or error output, but I don't see one.  Any suggestions?

Thank you SO much!

Eric

The (beginning of) the modpack manifest.json:

{
  "minecraft": {
    "version": "1.12.2",
    "modLoaders": [
      {
        "id": "forge-14.23.5.2854",
        "primary": true
      }
    ]
  },
  "manifestType": "minecraftModpack",
  "manifestVersion": 1,
  "name": "Avatar the Last Minebender",
  "version": "1.0.0",
  "author": "stupidjrob",
  "files": [
    {
      "projectID": 254565,
      
      ...

Here are all the files in my "universe" directory on the server:

<me>@game-server-1:/home/gameserver/minecraft/u/avatar_forge_1.12.2$ find . | grep -v 'saves'
.
./patchouli_data.json
./modlist.html
./llibrary
./llibrary/lang
./llibrary/debug
./llibrary/debug/net.minecraft.client.renderer.EntityRenderer.class
./llibrary/debug/net.minecraft.client.renderer.entity.RenderPlayer.class
./llibrary/debug/net.minecraft.client.model.ModelBiped.class
./llibrary/debug/net.minecraft.client.renderer.entity.RenderLivingBase.class
./llibrary/debug/net.minecraft.server.MinecraftServer.class
./llibrary/debug/net.minecraftforge.client.ForgeHooksClient.class
./llibrary/debug/net.minecraft.network.NetHandlerPlayServer.class
./llibrary/debug/net.minecraft.client.resources.Locale.class
./llibrary/debug/net.minecraft.client.renderer.tileentity.TileEntityItemStackRenderer.class
./llibrary/config.dat
./server-resource-packs
./usernamecache.json
./manifest.json
./world
./world/DIM-1
./world/session.lock
./world/data
./world/data/Mineshaft.dat
./world/data/villages_nether.dat
./world/data/villages.dat
./world/data/functions
./world/data/Village.dat
./world/data/villages_end.dat
./world/data/Temple.dat
./world/data/advancements
./world/level.dat
./world/playerdata
./world/playerdata/71abe94f-c105-4c55-83d5-c12e1f582e0d.dat
./world/playerdata/5e2ca1f6-27d3-4581-ae14-b1838569b170.dat
./world/DIM1
./world/region
./world/region/r.-1.0.mca
./world/region/r.0.0.mca
./world/advancements
./world/advancements/71abe94f-c105-4c55-83d5-c12e1f582e0d.json
./world/advancements/5e2ca1f6-27d3-4581-ae14-b1838569b170.json
./world/level.dat_old
./world/stats
./world/stats/71abe94f-c105-4c55-83d5-c12e1f582e0d.json
./world/stats/5e2ca1f6-27d3-4581-ae14-b1838569b170.json
./options.txt
./minecraftinstance.json
./journeymap
./journeymap/server
./journeymap/server/5.7
./journeymap/server/5.7/journeymap.server.default.config
./journeymap/server/5.7/journeymap.server.dim1.config
./journeymap/server/5.7/journeymap.server.global.config
./journeymap/server/5.7/journeymap.server.dim0.config
./journeymap/server/5.7/journeymap.server.dim-1.config
./journeymap/server/5.7/journeymap.server.dim7.config
./journeymap/server/5.7/journeymap.server.dim20.config
./journeymap/icon
./journeymap/icon/entity
./journeymap/icon/theme
./journeymap/icon/theme/default.theme.config
./journeymap/icon/theme/flat
./journeymap/icon/theme/flat/Stronghold.theme2.json
./journeymap/icon/theme/flat/DesertTemple.theme2.json
./journeymap/icon/theme/flat/NetherFortress.theme2.json
./journeymap/icon/theme/flat/OceanMonument.theme2.json
./journeymap/icon/theme/flat/EndCity.theme2.json
./journeymap/icon/theme/flat/Purist.theme2.json
./journeymap/icon/theme/flat/ForestMansion.theme2.json
./journeymap/data
./journeymap/data/sp
./journeymap/data/sp/AvatarTest
./journeymap/data/sp/AvatarTest/DIM0
./journeymap/data/sp/AvatarTest/DIM0/3
./journeymap/data/sp/AvatarTest/DIM0/3/-1,-1.png
./journeymap/data/sp/AvatarTest/DIM0/3/0,-1.png
./journeymap/data/sp/AvatarTest/DIM0/night
./journeymap/data/sp/AvatarTest/DIM0/night/-1,-1.png
./journeymap/data/sp/AvatarTest/DIM0/night/0,-1.png
./journeymap/data/sp/AvatarTest/DIM0/day
./journeymap/data/sp/AvatarTest/DIM0/day/-1,-1.png
./journeymap/data/sp/AvatarTest/DIM0/day/0,-1.png
./journeymap/data/sp/AvatarTest/DIM0/topo
./journeymap/data/sp/AvatarTest/DIM0/topo/-1,-1.png
./journeymap/data/sp/AvatarTest/DIM0/topo/0,-1.png
./journeymap/data/sp/AvatarTest/waypoints
./journeymap/journeymap.log
./journeymap/config
./journeymap/config/5.7
./journeymap/config/5.7/journeymap.topo.config
./journeymap/config/5.7/journeymap.webmap.config
./journeymap/config/5.7/journeymap.minimap.config
./journeymap/config/5.7/journeymap.core.config
./journeymap/config/5.7/journeymap.fullmap.config
./journeymap/config/5.7/journeymap.waypoint.config
./journeymap/config/5.7/journeymap.minimap2.config
./betweenlands_gallery
./betweenlands_gallery/gallery_1.0.0
./betweenlands_gallery/gallery_1.0.0/91167d3a463e19fe87947168d99a40f163b8f83e2c44d16305f204958ab1442f.png
./betweenlands_gallery/gallery_1.0.0/f1cd1f9b22ceb897c78c900d64fb906c475b900609da8efbc7a50727ffe77c8e.png
./betweenlands_gallery/gallery_1.0.0/a31e01d48a72542fb4307dcbc0b77a8a72d106e91558fd3e8b639944b8f76e48.png
./betweenlands_gallery/gallery_1.0.0/c18e0e21f09c25431a6807681c66705e7e01d35c398399e2a7e955cb84e697d9.png
./betweenlands_gallery/gallery_1.0.0/3bae4cd5bb1f934037a44a31a3c2010955cf5855270df7f2f5691612293daee3.png
./betweenlands_gallery/gallery_1.0.0/7a8d4997c96fcff9be90979fcf38490604084c1f49f245bef0c732b997ba3fee.png
./betweenlands_gallery/gallery_1.0.0/df06b00dc2a5d3ba9fd3ba1df504c818248c23682ec819d80e7135da8977a2a4.png
./betweenlands_gallery/gallery_1.0.0/569be77afc3e2545d9a2781818376cd1254f4a03f0031f347e419e1b48e43a7f.png
./betweenlands_gallery/gallery_1.0.0/5a5940ee603905f3e045e2de5eb4c4489fcf3bb09e05b22b9992caa00ace981d.png
./betweenlands_gallery/gallery_1.0.0/6fc2f76026fe1e91bb1fd60686d2fde6046aea481518a737e57bb11dc70786f5.png
./betweenlands_gallery/gallery_1.0.0/158faf03e64080e470a089a04be9cb735f1781321702bba82812349f99039b5b.png
./betweenlands_gallery/gallery_1.0.0/03522b748767a80a268013354201f8a4730f7c225a0d859ed920a76c82cddcad.png
./betweenlands_gallery/gallery_1.0.0/e99ba84ccc545bbcbbff3bf747bc62bc1d4cb6534ce06130553f9eb2f14f3222.png
./betweenlands_gallery/gallery_1.0.0/0dd3bd5300693cea2d1e10d5363553bd0206fe8295e46954f249523dcd8f26ab.png
./betweenlands_gallery/gallery_1.0.0/8c8cbb4ce7a12b9ced4245fcefac64901a46feb62cfe4926294fbf6606827729.png
./betweenlands_gallery/gallery_1.0.0/36be6a65f99975905ba1bebd265a19ae13058d50e67cace20e048756341121ee.png
./betweenlands_gallery/gallery_1.0.0/93b40afc564405b4208edff2d6cfb0ee421b7bf842f06df7bf866c61034d17c7.png
./betweenlands_gallery/gallery_1.0.0/4b0c6fc1a7722db2679aafd6a0f6d021842dcbcce1768f1adac9e67aa9113d09.png
./betweenlands_gallery/gallery_1.0.0/49030579384c3a2258b38f45d18c1f8da04c35b57842736a89e5722d0542af14.png
./betweenlands_gallery/gallery_1.0.0/5b6b0ff70e71d2efa99849081e85dbc3d703fa78f31cde107edc2a2677848136.png
./betweenlands_gallery/gallery_1.0.0/78e99ea831a3272d1ddedec8d6ecfbf7af41997b1e095d07ea9b0ecbb24038ae.png
./betweenlands_gallery/gallery_1.0.0/index.json
./betweenlands_gallery/gallery_1.0.0/422129a4ba59114bbb854e625bcc7ba8dc6dd9545ebbb1351b3f45eec8f3ca90.png
./betweenlands_gallery/gallery_1.0.0/050a38d5b4638bd9f0f9c6c5729d42a8cec62265c25089da82964676f46e3e16.png
./betweenlands_gallery/gallery_1.0.0/93d8fa98bf85281fe5034e608019b387350f38414f16b7849f52e182286a0c8d.png
./betweenlands_gallery/gallery_1.0.0/9728ca616b703784874bf7e8cd8cf5060fceead7e116c4f26af20f513d666d88.png
./betweenlands_gallery/gallery_1.0.0/b32dfcb07188e23688f646b79fbce0e36be584e25952c13b9726f73673aa29fb.png
./config
./config/splash.properties
./config/ctm.cfg
./config/forgeChunkLoading.cfg
./config/netherex
./config/netherex/netherex.cfg
./config/infernalmobs.cfg
./config/jei
./config/jei/worldSettings.cfg
./config/jei/searchColors.cfg
./config/jei/jei.cfg
./config/jei/itemBlacklist.cfg
./config/journeymap_server.cfg
./config/morpheus.cfg
./config/bloodmoon.cfg
./config/roguelike_dungeons
./config/roguelike_dungeons/settings
./config/roguelike_dungeons/roguelike.cfg
./config/journeymap_ModInfo.cfg
./config/ice_and_fire.cfg
./config/hiddenarmor.cfg
./config/gorecore.cfg
./config/neat.cfg
./config/forge.cfg
./config/mowziesmobs.cfg
./config/thebetweenlands
./config/thebetweenlands/config_version
./config/thebetweenlands/config.cfg
./config/twilightforest.cfg
./config/patchouli.cfg
./config/explorercraft
./config/explorercraft/explorercraft.cfg
./config/avatar
./config/avatar/cosmetic.yml
./config/avatar/analytics.yml
./config/avatar/stats.yml
./config/avatar/skills.yml
./config/avatar/mobs.yml
./config/avatar/chi.yml
./mods
./mods/Patchouli-1.0-20.jar
./mods/twilightforest-1.12.2-3.10.1013-universal.jar
./mods/avatarmod-1.5.17-alpha.jar
./mods/journeymap-1.12.2-5.7.1.jar
./mods/jei_1.12.2-4.15.0.291.jar
./mods/RoguelikeDungeons-1.12.2-1.8.0.jar
./mods/TheBetweenlands-3.6.1-universal.jar
./mods/CTM-MC1.12.2-1.0.2.31.jar
./mods/Neat 1.4-17.jar
./mods/memory_repo
./mods/memory_repo/com
./mods/memory_repo/com/electronwill
./mods/memory_repo/com/electronwill/night-config
./mods/memory_repo/com/electronwill/night-config/core
./mods/memory_repo/com/electronwill/night-config/core/3.6.0
./mods/memory_repo/com/electronwill/night-config/core/3.6.0/core-3.6.0.jar
./mods/memory_repo/com/electronwill/night-config/core/3.6.0/core-3.6.0.jar.meta
./mods/memory_repo/com/electronwill/night-config/json
./mods/memory_repo/com/electronwill/night-config/json/3.6.0
./mods/memory_repo/com/electronwill/night-config/json/3.6.0/json-3.6.0.jar
./mods/memory_repo/com/electronwill/night-config/json/3.6.0/json-3.6.0.jar.meta
./mods/memory_repo/net
./mods/memory_repo/net/ilexiconn
./mods/memory_repo/net/ilexiconn/llibrary-core
./mods/memory_repo/net/ilexiconn/llibrary-core/1.0.11-1.12.2
./mods/memory_repo/net/ilexiconn/llibrary-core/1.0.11-1.12.2/llibrary-core-1.0.11-1.12.2.jar.meta
./mods/memory_repo/net/ilexiconn/llibrary-core/1.0.11-1.12.2/llibrary-core-1.0.11-1.12.2.jar
./mods/memory_repo/angrypixel
./mods/memory_repo/angrypixel/TheBetweenlands
./mods/memory_repo/angrypixel/TheBetweenlands/3.6.1
./mods/memory_repo/angrypixel/TheBetweenlands/3.6.1/TheBetweenlands-3.6.1-core.jar.meta
./mods/memory_repo/angrypixel/TheBetweenlands/3.6.1/TheBetweenlands-3.6.1-core.jar
./mods/MouseTweaks-2.10-mc1.12.2.jar
./mods/InfernalMobs-1.12.2.jar
./mods/iceandfire-1.9.1-1.12.2.jar
./mods/Morpheus-1.12.2-3.5.106.jar
./mods/mowziesmobs-1.5.5.jar
./mods/LibraryEx-1.12.2-1.1.3.jar
./mods/hiddenarmor-1.12.2-1.0.jar
./mods/llibrary-1.7.19-1.12.2.jar
./mods/Explorercraft-2.2.14.jar
./mods/NetherEx-1.12.2-2.1.7.jar
./mods/Bloodmoon-MC1.12.2-1.5.3.jar
./usercache.json
./downloads
./resourcepacks
./patchouli_books

Edited July 26, 20205 yr by Eric A.
Added that this is a Linux server