Thanks to the quick reply!
I have noted after your message that the page loads with the port 8002. But then gets lost when trying to load on the port 8000.
EDIT:
Your post showed me the right direction. Indeed I had a mis configured Form in one of my Log in Pages.
I had the complete address including the port 8000 as a target instead of just the variable.
This however seemed to work till the last Version but the upgrade process seemed to not light this.
Thanks for your help!