Thank you both a lot, theducks, kovidgoyal!
Much appreciated!
(I didn't know about the dedicated tmp folder, I think I'll do just that.)
I don't quite understand why the editor had trouble accessing the tmp folder, since it never happened before. A restart of my notebook however, took care of that problem.
Mind, I had to restart *twice*, because of this strange behaviour:
The first time I restarted, I immediately opened the editor and the epub in question, clicked on "import files into book", added one of the html in question, and voila, it worked fine. I then opened calibre, but before I actually did anything in calibre, I had an idea and went back to the editor to add the other html files to the new ebook, but this time I basically got the same error as above. Since I had absolutely no other program open, I have to assume that it was calibre that blocked the tmp folder. I restarted again, but this time I could not reproduce this behaviour, and in fact I now have no problem adding even multiple files into the ebook.