Thanks a lot for the very helpful comments (I didn't get notifications about further replies, that's why I just read them...)!
I guess the problem really is the model which is a relief because it was just a random downloaded humanoid model for testing.
Moreover, I just found that the FBX import allows for reducing the number of frames per second so that instead of using all 40 frames of a JimmyRig walkcycle I can only use 6-8 which makes things tidier.
Thx again for the clarification.