I've been able to reproduce the Bug. Not quite sure how I did it and what is triggering it but I'm optimistic to fix it for next version.
I've done some investigation on the hang I've discovered. In my case animals mod somehow seems to trigger a bug in minetest core making one of it's threads throwing an exception on collision handling. I'm not sure if I'll be able to fix that in animals mod itself. Probably it's a sideeffect of the "wielditem" abuse.
Ok I think I've got it. Problem isn't exactly the exception itself but the number of exceptions. But start from the beginning.
First the facts:
1) map ain't loaded in client anymore
2) animals are visible on client without terrain
3) exceptions on server occur due to animals "colliding" with unloaded nodes
4) "emerge thread" loading terrain doesn't get active until minetest crashes after some time.
What I think is happening:
Entitys beeing loaded prior map beeing loaded wasn't really a situation I expected when programming animals mod. Many functions rely on map data beeing available and VALID.
In follow up movement is started somehow and then simply overloading minetest servers collision handling.
on_activate checks if node animal is is really loaded and delays activation of on_step handling until map is really loaded.
I'm doing some investigations if this really fixes the problem and release a new version of animals mod soon.