I had a few print strings set up, with 0 duration, which ive connected to the event tick.
Suddenly, the print string did only show, when the window is not focused.
Usually i fixed it by adding a print string in the event tick of the parent class, compiled and removed it again.
My other functions seem to work properly in the event tick.
Having the physic asset opened on a second monitor (or while in background) somehow disabled it.
Closing those windows brought back the proper functionality of setting print string duration to 0.