Dear Tom,
I found the problem.
It never left me alone again.
My settings were as follows:
Permalink Settings => Custom Structure => domain.com /%category%/%postname%/ (no matter which parameters)
When I change the setting to:
Permalink Settings => Post name => domain.com/sample-post/ (then it works)
I won’t understand why that’s the case. Is wordpress ๐
But I want to thank you for your help!
Your support is great
Ron