Thanks for your efforts! But I think I've narrowed the problem to my rx-51 keyboard configuration. I changed it a bit (added < > | ) which worked fine so far but maybe there is some side effect that didn't appear up to now.
This morning I noticed that Shift+Enter works mostly* fine. I'll restore the original rx-51 file later and see if that changes anything.

*) mostly means that Shift+Enter works instantly if I haven't pressed Enter in vfu before, that I have to hold Shift and press Enter twice if I have pressed Enter alone once and that I have to hold Shift and press Enter thrice if I've pressed Enter alone two or more times before.
Please don't ask me for an explanation of this behavior!