I've searched in the code and I haven't found what is precisely this error message. I haven't tested the new version of animals modpack yet.
There is a file called debug.txt in MT's files in which you can find the error logs. That must help us understanding the problem. If the file is too big, remove it and launch Minetest again with animal_modpack. So you will find a new debug.txt with only the informations we need.
Or you can try other mobs mods like
Mobs Redo, that is much lighter.
Very busy this year too, so do not expect me to be very active on the forum or in game. But I'm not about to drop Minetest forever :)