Intro:
Hi, this mod is great! When it works..
When I was playing the modpack only in Singleplayer, this was a blessing. But seems to never work when Joining a Server.

Issue:
Stuck on Registering Categories when joining a Server, and the game never loads past Joining World.
I've waited up to 15m in the past to see if the game ever loads.

• Issue seems to resolve after a couple minutes on Single Player worlds, but never concludes when joining a Server.

Reproduction Steps:

1. Install Fantasy MC Fabric (any version but currently playing v2.4.2 which is the latest)
2. Install JEI and JER
3. Join any Server

Example Partial Log:
[16:55:30] [Render thread/INFO]: [bclib] Folders offered by Server:
[16:55:30] [Render thread/INFO]: [bclib] - BCLIB-SYNC (D:\Windows\Games\Minecraft\MultiMC\MultiMC\instances\Fantasy MC Fabric.minecraft\bclib-sync, allowRemove=true)
[16:55:32] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 10.00 seconds so far
[16:55:37] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 15.00 seconds so far
[16:55:42] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 20.00 seconds so far
[16:55:47] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 25.00 seconds so far
[16:55:52] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 30.00 seconds so far
[16:55:57] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 35.00 seconds so far
[16:56:02] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 40.00 seconds so far
[16:56:07] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 45.00 seconds so far
[16:56:12] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 50.00 seconds so far
[16:56:17] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 55.00 seconds so far
[16:56:22] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.000 minutes so far
[16:56:27] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.083 minutes so far
[16:56:32] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.167 minutes so far
[16:56:37] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.250 minutes so far
[16:56:42] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.333 minutes so far
[16:56:47] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.417 minutes so far
[16:56:52] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.500 minutes so far
[16:56:57] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.583 minutes so far
[16:57:02] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.667 minutes so far
[16:57:07] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.750 minutes so far
[16:57:12] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.833 minutes so far
[16:57:17] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 1.917 minutes so far
[16:57:22] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.000 minutes so far
[16:57:27] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.083 minutes so far
[16:57:32] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.167 minutes so far
[16:57:37] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.250 minutes so far
[16:57:42] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.333 minutes so far
[16:57:47] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.417 minutes so far
[16:57:52] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.500 minutes so far
[16:57:57] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.583 minutes so far
[16:58:02] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.667 minutes so far
[16:58:07] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.750 minutes so far
[16:58:12] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.833 minutes so far
[16:58:17] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 2.917 minutes so far
[16:58:22] [pool-30-thread-1/ERROR]: Registering categories: minecraft:jeresources is running and has taken 3.000 minutes so far

(No mention of a specific error occurs in log.. since it gets stuck Registering Categories.. No obvious sign of another mod causing this before the Register Categories, everything else seems to load fine.)

Same happens to me on 1.20.1 Fabric with a custom modpack. I'll try looking for mods we have in common, maybe some of them could be causing the issue...

Same happens to me on 1.20.1 Fabric with a custom modpack. I'll try looking for mods we have in common, maybe some of them could be causing the issue...

Yea I've done a deep dive in the past on this, disabling / removing mods, trying to see if the issue gets fixed.. but couldn't find any one or few causing the issue..
Might be that the modpack has many many mods, 400+ mods.. and it gets caught trying to scan and register them all.
But like I said in the Intro, the mod Does Work in Singleplayer... Just not on servers.. Which is interesting to say the least.

Exactly the same happens to me on my 1.20.1 Quilt custom modpack. The issue is on my Arch linux machine, but my two friends with the exact same modpack on windows seem to not have it. Maybe that's the problem?
Singleplayer loads with no delays whatsoever, only multiplayer is the issue. I haven't tested LAN though, in case it is dedicated-server related (although unlikely given the circumstances).
EDIT: okay my bad, just tested with more friends and it didn't work for most of them as well, so not a linux issue sadly :(

Had this issue while making a custom modpack, and it seems to be caused by a conflict between JER and C2ME. Unsure what the conflict is, but the presence of C2ME causes the game to get stuck in an infinite loop registering categories...

Had this issue while making a custom modpack, and it seems to be caused by a conflict between JER and C2ME. Unsure what the conflict is, but the presence of C2ME causes the game to get stuck in an infinite loop registering categories...

You are the best! Had a server with 200 mods that worked perfectly locally but not hosted. Removed changed C2ME in the client and it worked perfectly!