I am debugging recently E2EE ZS mixin changes.
I found this like in dedicated server's crafttweaker.log:
[CONSTRUCTING][SERVER][ERROR] [mixin | SIDE_SERVER]: Error parsing mixin/oldjavawarning.zs:4 -- class net.darkhax.oldjava.OldJavaWarning does not exist.
Despite the fact that other mixins not reporting any errors and considered as "working" I still can see non-changed behaviour, for example,
[CONSTRUCTING][SERVER][TRACE] [mixin | SIDE_SERVER]: Loading Script: {[0:mixin]: mixin/tconstruct.zs}
[16:51:35] [Server thread/INFO] [tconstruct-TinkerSmeltery]: Started adding oredict melting recipes
Adding #sideonly client to the oldjavawarning.zs ofc fixes the issue, but it would be more oblivious if erroring script not break all the mixins.