Hey Dirk,
Thanks for opening up a ticket like we requested in that other ticket – I know it can seem petty from the outside, but it helps us stay organized and handle each issue on their own, which is helpful because similar-looking problems may not have the same source.
In your case, for example, the biggest red flag that stands out is the following statement of yours:
(On a clean install on a different domain we didn’t have the problem btw.)
That seems to indicate that there is indeed either something with your site’s redirect/rewrite rules that is causing the problems, or a theme or plugin conflict that somehow passed through your first round of testing.
I’m curious – what are the Permalinks settings on your site under Settings > Permalinks?
Do changing these settings affect this behavior at all?