In order to separate multiplayer animations from singleplayer ones, the next update is going to change the default animations for player models.

If you've made playermodels using default animations, they're going to break you'll need to recompile and replace them.

What's changing:
Instead of using animations in the humans/ folder, you should use new animations that will be in the player/ folder instead.

This means if your model looks like this now:

You should change it to this instead:

In the future a player model UI will only display models under models/player/ so it's strongly suggested you change your folder structure if they aren't there already.