Now this is interesting: I tried to reproduce the issue (by the way I'm quite sure I did try the clean-up as well), but couldn't. For one the ingest speed was as expected (so guess that was a glitch), but now also cancelling worked fine.
So, my guess is that the slow transfer kinda blocked PM (it was still busy transferring a file), and quitting would happen only after the remaining transfer finished (which took really very very long to complete). I guess I was just too impatient
Issue closed.
Cheers,
Hayo