Hi. I have made a few small changes to the mod to morph randomly every minute. And it worked fine in single-player but multiplayer doesn't work at all. The server crashes as soon as I join. The unchanged mod works fine, I hope it is okay to ask for help about this kind of stuff.

I use fabric-API-0.30.0

Here is the modification I made which causes the server to crash.

I added 2 functions to run on every tick

These are the two classes where I made changes:
https://github.com/xBogdy/minecraftMods/blob/75aa4bf248a4a8e83e5df00a164aa42aa8d118c4/src/main/java/draylar/identity/screen/widget/EntityWidget.java#L54

https://github.com/xBogdy/minecraftMods/blob/master/src/main/java/draylar/identity/cca/IdentityComponent.java

This is the server's crash report:
crash-2021-04-08_00.08.01-server.txt
and log:
latest.log
2021-04-07-6.log.gz

I'd greatly appreciate your help

You put your server utility methods inside a client-side class, hence the NoClassDefFoundError.

If you need random auto-swapping behavior, I just pushed the autoswap branch, which was made a month or two ago for some video content. You can build that branch for [swap every minute/swap every day/swap on damage] functionality and edit the new config options (for toggling functionality) or modify the code if you need additional changes.