Thanks for your post, at least it confirmed I did not do something really obvious and stupid. The scene is on the same quest but the error stems from something that is not visible in the code above, but in the code seen on the pastebin link. Sorry, I was on mobile, maybe it was not clear enough. I "solved" it though. Here is a possible explanation: Using setstage(xx) within a Quest Fragment seems to stop scenes from starting properly, if you use SetStage(B) on Stage A and want to start a Scene on Stage B. Or with code:
;DONT DO THIS!
;Quest stage 5 (A)
Debug.Notification("Hey we are on Stage 5")
utility.wait(1.0) ;just for good measure
Setstage(10)
;Quest stage 10 (B)
myScene.start()
If I cut the line that sets the stage to 10 and set the stage from somewhere else, then it works properly. The "cannot start scene because its parent quest was not running" is definitely misleading though. I understand that what I did - jumping from stage to stage - is not good practice, but I only used that in a brief test quest and it completely threw me off. I hope others won't waste as much time as I did.