This bug renders my server unplayable unless I quickly teleport myself out of the area.

After strolling through the nether, the pack attempted to place a feature with an invalid identifier name:

net.minecraft.util.InvalidIdentifierException: Non [a-z0-9/._-] character in path of location: repurposed_structures:Random spawn bonus

Repurposed Structures v7.5.1 for Fabric 1.21 (loader version 0.15.11)
Full crash log

This bug was introduced in 1.21. It happens when the game attempts to spawn a bowman wither skeleton, then modify it with a random spawn bonus attribute. However, the provided ID is not syntactically valid. What this is intended to modify is unclear.

I can't find the implementation for whatever this attribute ID is a placeholder for. Either it is unimplemented, or I'm not able to find it at a glance:

Thanks,

AttributeModifiers need an ID so the game adds it as a new entry onto the mob. It's not something you register. It's a replacement for the UUID mojang used to have for every time they added an attribute to a mob to distinguish each new AttributeModifier added to a mob or if it overrides an existing one of same ID.

Fixed now and released

Ah, interesting, that makes more sense. Thanks again!