Deleting mods from LAN on client breaks it

[c2oh25n3o]

Hi!

We're playing Minecraft 1.16.5, forge 36.1.0 with my friend on a client with opened LAN (I own the client save file) with a bunch of mods. Our setups are 100% the same, we use CurseForge launcher and have exactly the same profile.

I really wanted to delete a few mods to increase performance a little bit, but when I do so my friend cannot connect to the server - it says 'Failed To Synchronize Registry Data From Server'.

The overall situation is that the save it still somehow attached to the deleted mods - in the live log there are lot of "Found a missing id" lines related to them. 

Besides the missing objects, there is another thing going on - "Unknown structure start", which means that the game is still trying to create structures from deleted mods. I tried cleaning the /DIM-1/regions and /regions folder files with MCStructure cleaner https://github.com/Nyveon/MCStructureCleaner, and it did delete a lot of things there (according to the command prompt), but for some reason the game log shows that those structures are still present, I don't know how that works. But I think that even if I managed to remove missing structures, the missing blocks and entities would still corrupt the save and it would still be impossible to connect. 

So I just really wanna know if there are some solutions to that problem. Is there any way to delete world-changing mods from LAN completely? Or that means once I installed them there is no way to go back?
I've found some information about using a "--forceUpgrade" option when launching the server to upgrade the world, but I'm not sure if it works on client games with open LAN (and if it works in general).

To be more certain, I'm trying to delete these mods: Mushroom Quest, Slimier Slimes, Fins and Tails. I think the first 2 mods are the most problematic because they add new blocks to the world generation. But I used to face the same issue when deleted different other mods in the past, they all just change the world somehow. Removing utility mods (e.g. Performant, Ambient Sounds, Carry on) obviously doesn't cause any trouble (as well as removing some small mods adding items, like Artifacts, Magic Mirror, TorchBow, AntiqueAtlas etc.)

I know I should provide both log files but the one I have is 6x times bigger than I can upload so below I'll put some of the lines appearing in it (there are a LOT of similar ones - I didn't include all of them), and my friend's log file is here.

Spoiler

(↓MULTIPLY ALL OF THEM BY 100)

[09:28:21] [Render thread/INFO]: Injecting existing registry data into this CLIENT instance
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world fins:chained_tentacle
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world fins:chiseled_flatback_shell_bricks
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world fins:chiseled_pebble_shell_bricks
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world fins:spindly_gem_crab_gem_block
...
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world mushroomquest:acaciastump
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world mushroomquest:baobabtree
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world mushroomquest:bigmushroombunch
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world mushroomquest:puffballs
...
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world slimier-slimes:amethyst_cluster
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world slimier-slimes:black_slime_block
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world slimier-slimes:black_slime_block_button
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world slimier-slimes:black_slime_block_pressure_plate
[09:28:21] [Render thread/INFO]: Registry Block: Found a missing id from the world slimier-slimes:black_slime_block_slab
...

---------------------------------------------------------

[09:28:22] [Render thread/ERROR]: Unidentified mapping from registry minecraft:block
    fins:mixed_pebble_shell_bricks: 6677
    fins:pebble_shell_brick_slab: 6668
    fins:pebble_shell_brick_stairs: 6669
    fins:pebble_shell_bricks: 6670
    fins:siderol_shell_brick_stairs: 6674
    fins:siderol_shell_bricks: 6675
    fins:spindly_gem_crab_gem_block: 6659
    ...
    mushroomquest:glowstoolbunch: 2749
    mushroomquest:greenglowshroom: 2740
    mushroomquest:jungletrunk: 2750
    mushroomquest:koru: 2756
    mushroomquest:lanternshroomblock: 2736
    mushroomquest:magentaglowshroom: 2739
    mushroomquest:myceliumkoru: 2757
    mushroomquest:myceliumshortgrass: 2744
    mushroomquest:myceliumshrump: 2746
    ...
    slimier-slimes:amethyst_cluster: 1375
    slimier-slimes:black_slime_block: 1350
    slimier-slimes:black_slime_block_button: 1353
    slimier-slimes:black_slime_block_pressure_plate: 1354
    slimier-slimes:black_slime_block_slab: 1351
    slimier-slimes:black_slime_block_stairs: 1352
    slimier-slimes:black_slime_block_wall: 1355
    ...

---------------------------------------------------------

[09:28:22] [Render thread/WARN]: Forge Mod Loader detected missing registry entries.

There are 464 missing entries in this save.
If you continue the missing entries will get removed.
A world backup will be automatically created in your saves directory.

[09:28:22] [Render thread/WARN]: Missing minecraft:block:
    fins:chained_tentacle
    fins:chiseled_flatback_shell_bricks
    fins:chiseled_pebble_shell_bricks
    fins:chiseled_siderol_shell_bricks
    fins:pebble_shell_pillar
    fins:siderol_shell_brick_slab
    fins:siderol_shell_brick_stairs
    fins:siderol_shell_bricks
    fins:spindly_gem_crab_gem_block

    ...
    mushroomquest:acaciastump
    mushroomquest:fan_fern
    mushroomquest:fan_fern_block
    mushroomquest:fieldtrunk
    mushroomquest:willowtree
    mushroomquest:witchurn
    mushroomquest:ygdrassilsapling

    ...
    slimier-slimes:amethyst_cluster
    slimier-slimes:black_slime_block
    slimier-slimes:black_slime_block_button
    slimier-slimes:black_slime_block_wall
    slimier-slimes:black_slimy_cobblestone
    slimier-slimes:black_slimy_cobblestone_button

    ...

---------------------------------------------------------

[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon
[09:29:41] [Server thread/ERROR]: Unknown structure start: slimier-slimes:slime_dungeon

...

---------------------------------------------------------

[09:30:19] [Render thread/INFO]: [CHAT] Local game hosted on port 56302
[09:30:24] [User Authenticator #1/INFO]: UUID of player Balastogur is cd39604a-2122-4bce-a308-0583d16fabe9
[09:30:26] [Server thread/INFO]: com.mojang.authlib.GameProfile@21777c31[id=cd39604a-2122-4bce-a308-0583d16fabe9,name=Balastogur,properties={textures=[com.mojang.authlib.properties.Property@63a7a87e]},legacy=false] (/192.168.1.38:51432) lost connection: Disconnected

 

Edited March 25, 2021 by c2oh25n3o