All is well now!
I couldn't test that properly, because calibre started with fetch news jobs which would not stop when I tried to abort them. So my conversion was stuck in the queue.
When I'd deselected the news sources and restarted calibre, I was informed of an available update. In the info to your update I didn't see something I found directly addressing this problem, but updated nevertheless to calibre 0.8.58 and started calibre.
At. which. point. fetch. news. worked. perfectly.
Just like that. Thanks a lot for your patience and support! Sometimes, fairy tales come true.
Greetings from Brother Grimm-city Hanau, Germany!