MC 1.7.10
Mowzies Mobs 1.7.10-1.2.99

Not sure if this is exactly a Mowzies Mobs issue, seems like it uses StructureGenerationAPI for structure generation.
But I got this crash out of the blue and I don't have StructureGenAPI installed separately as a mod.

crash-2021-11-10_12.22.24-server.txt

StructureGenAPI was merged into LLibrary.

Sorry, but I don’t support this version of the mod anymore…