Help please: connection issues Windows - ISP (Ziggo) Netherlands

Hi, I'm trying to solve an issue I've been experiencing for many months. It is not browser related (happens in all), it's not ethernet cable or Wifi related (happens in both), it is not specific website related (it also happens in Power Automate, VSC and my Voice ISP (which shows even worse issues)).

The problem (seems) is when the app sends a request which includes parameters to a website, even like office.com/?auth=2 or /login?redirect=/get-started or kind like parameters there seem to be issues.

Windows 11 / Dell quality pc / good cables / modern router - coax in house, fiber to the building.

Checked everything, factory reset everything many times, the problems occur again and again and after many talks and visits from Ziggo ISP the problem remains.

Any clues would be welcome!

Thanks, Martin