I have subbed to the new content pack and it made basically all of my playermodels to A-Pose, float etc.
My best theory out of the top of my head is that something in the content pack conflicts with regular GMod animations. The only ones that didnt conflict was Chell, Alyx, Dr. Moosman (both versions), Zombie and Fast Zombie.
I got these types of errors
'player/skeleton.mdl' animation requested bone 122, but model only has 56!'