https://pastebin.com/smWUXG04

SpongePowered/SpongeForge#1491

It turned out to be some corrupted data, but I'll leave this open anyways, just in case something else can be gleamed from the error.

file handling gets some additional safety mechanisms to prevent this from happening. If data is unrecoverably lost, i can't do much about it...

Unfortunately, this happens if it's in the middle of writing the data to the file while the server crashes or similiar. It's stored inside a directory in the world's directory so it should get backup'd when creating a world backup.

Regardless, this is most likely adressed as of next version.