Transition doesnt happen, although rule ist met

I am currently working on an animation blueprint, based on the shooter demo character. I created a new state in the state machine and created a transition rule, which just takes a (defined by code) boolean. But even though this one is true, the transition never happens in game (but perfectly at the preview instance)…
At first I thought, it would be some server/client issue, but every other transition in the blueprint just works fine.
Anyone else having the same problem or an idea what could cause this error?