I have an FML 2847 server setup. So far, only two people are using the server (both of us are opped and whitelisted with whitelist turned on).

 

We cannot place items in creative mode. In survival mode, there's no problem. Only the client crashes. The server just notes that the player has disconnected.

 

Here's my latest.log, Crash Report, and the server log.

 

I was initially on FML 2768, but in an attempt to fix this problem, I updated to the latest versions of all the mods and that required I update to 2847. All mods were directly downloaded from either curseforge or the modder's own site. Although, strangely, JourneyMap site has an older version than curseforge, so I went with the curseforge version.

 

Just to be thorough, I also deleted all config files on both the server-side and client-side.

 

My mod list (identical on both server and client side):

 

Spoiler

BiomesOPlenty-1.12.2-7.0.1.2441-universal.jar
cfm-6.3.0-mc1.12.2.jar
Chameleon-1.12-4.1.3.jar
chiselsandbits-14.33.jar
creeperheal-1.12-1.3.1.jar
Dynmap-3.0-beta-4-forge-1.12.2.jar
EmberRootZoo-1.12.2-1.3.10.jar
fairylights-2.1.9-1.12.2.jar
forgeessentials-1.12.2-12.3.54-client.jar
forgeessentials-1.12.2-12.3.70-server.jar
Hwyla-1.8.26-B41_1.12.2.jar
iceandfire-1.8.3-1.12.2.jar
inventorypets-1.12-2.0.3.jar
InventoryTweaks-1.63-1.12.2.jar
jei_1.12.2-4.15.0.291.jar
journeymap-1.12.2-5.5.5.jar
llibrary-1.7.19-1.12.2.jar
Mantle-1.12-1.3.3.55.jar
Mo-Pickaxes-Mod-1.12.2.jar
Pam's+HarvestCraft+1.12.2zf.jar
StorageDrawers-1.12.2-5.4.0.jar
TConstruct-1.12.2-2.12.0.157.jar
TombManyGraves-1.12-4.2.0.jar
TreeChopper-1.12.2-1.2.4.jar
ultimate_unicorn_mod-1.12.2-1.5.16.jar
Weee!+Flowers+1.12.2b.jar
wings-1.1.6-1.12.2.jar
WorldEdit+CUI+Forge+Edition+2-2.2.0-mf-1.12.2-14.23.5.2768.jar

 

I fixed this. I had both client and server installed on the server for ForgeEssentials

 

forgeessentials-1.12.2-12.3.54-client.jar
forgeessentials-1.12.2-12.3.70-server.jar

 

I removed the client and all is fine.