Last night after changing some ID's in my mod, I randomly started to get this error whenever I traveled to my dimension.
In it says that is not recoverable, and I edited base classes. I never edited any base classes and in fact backed up my source files, and completely made a new entire forge, mcp folder. I still get that error though. (I also changed the ID's back to their original numbers.)
It had to do with my ChunkProvider, and Biome. I have it all fixed, thanks for any one who read it.