No access to Kirby panel

I can’t access anymore the Kirby panel.
The message ist “NetworkError when attempting to fetch resource.”
The DNS is hosted on cloudflare since a few months, the web space is hosted on IONOS.
Since yesterday I haven’t anymore access. How to solve /debug this, what could be the reason?
The website works fine as always, I just haven’t access.

This seems to be a CORS issue as the browser tries to fetch resources from instead of