Steps to Reproduce the Crash

When anything past 19.2.8.99 is installed server side, server refuses to start with the following errors.

This isn't just you btw. I've reported it for Blockus and Polymer in the last 48 hours.

Mod Pack URL (Optional)

No response

Mod Pack Version (Optional)

No response

Extra Notes (Optional)

No response

Crash Report

https://gist.github.com/Wytchcat/63435c61bd77321b40c8736fa900dc44

Thanks for the report!
I have pushed a fix that should allow JEI 1.21.1 to work with Minecraft 1.21, it will be available shortly.
Please let me know if you run into any more issues.

20:14:52 [Info] Immediate reason: [HARD_DEP_NO_CANDIDATE jei x.x.x.192 {depends fabricloader @ [fabricLoaderVersionRange]}, HARD_DEP_NO_CANDIDATE jei 19.16.5.185 {depends fabric-api @ [>=0.103.0+1.21.1]}, ROOT_FORCELOAD jei]
20:14:52 [Info] Reason: [HARD_DEP jei x.x.x.185 {depends fabric-api @ [>=0.103.0+1.21.1]}]
20:14:52 [Info] SERVER environment disabled: [fabric-model-loading-api-v1, fabric-renderer-api-v1, fabric-keybindings-v0, modmenu, fabric-client-tags-api-v1, nemos-ambience, fabric-screen-api-v1, nether-coords, fabric-renderer-indigo, fabric-blockrenderlayer-v1, fabric-renderer-registries-v1, fabric-rendering-v0, fabric-sound-api-v1, fabric-rendering-v1, fabric-key-binding-api-v1]
20:14:52 [Info] Fix: add [], remove [], replace [[jei x.x.x.192, jei 19.16.5.185] -> add:jei 1 ([(-∞,∞)])]
20:14:52 [Error] Incompatible mods found!
net.fabricmc.loader.impl.FormattedException: Some of your mods are incompatible with the game or each other!
A potential solution has been determined, this may resolve your problem:

• Replace mod 'Just Enough Items' (jei) x.x.x.185 and mod 'Just Enough Items' (jei) 19.17.0.192 with mod 'Just Enough Items' (jei), any version.
  More details:
• Mod 'Just Enough Items' (jei) x.x.x.185 requires version 0.103.0+1.21.1 or later of mod 'Fabric API' (fabric-api), but only the wrong version is present: 0.102.0+1.21!
  at net.fabricmc.loader.impl.FormattedException.ofLocalized(FormattedException.java:51) ~[fabric-loader-0.16.3.jar:?]
  at net.fabricmc.loader.impl.FabricLoaderImpl.load(FabricLoaderImpl.java:196) ~[fabric-loader-0.16.3.jar:?]
  at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:146) ~[fabric-loader-0.16.3.jar:?]
  at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68) [fabric-loader-0.16.3.jar:?]
  at net.fabricmc.loader.impl.launch.knot.KnotServer.main(KnotServer.java:23) [fabric-loader-0.16.3.jar:?]
  at net.fabricmc.loader.impl.launch.server.FabricServerLauncher.main(FabricServerLauncher.java:69) [fabric-loader-0.16.3.jar:?]
  20:14:52 [Info] [net.creeperhost.wailt.web.ApiWebServer]: Stopping web server..

Fabric loader for 1.21 is still 102.0

It looks like you have multiple versions of JEI installed, both 19.17.0.192 and the older version 19.16.5.185 are shown.

See open report:
#3750 (comment)

includes updated server crash report. Thanks