Hi! Thanks for the replies! I tried switching to box2d asm but the problem persisted. Might be a logic error too, I'm making those all the time.
The animation in which the spring contracts is not playing, unless you set the speed of the "backwardsTension"-animation to 60.
Could this have something to do with dt?
EDIT:
I think it's a logic error, will troubleshoot some more on my own