Deformed geometry when playing animation

I received a model from my 3d artist, including some animations and I integrated it into the project, but I found that, when the animation is played, the geometry is deformed:

If no animation is played, the model geometry looks good. During export, I have ensured that no errors are displayed (first I got one about needing to apply scale to armature), except for one that says something about vertex incompatible with indices. Can somebody see whats going wrong here?