console log: https://paste.ee/p/roLSH
no other crash files available

That log message is a false positive, and will be fixed in the next ModernFix release. It's related to the hacks I do to replace part of the Forge classloading system with a faster code path.

The game crash should be unrelated to the Illegal prefix specified for serializationisbad.

But I will take a look into the cause of that log message.

any update regarding the issue? do you need more information?