Unable to login to production instance: Your authorization token has expired

We discovered this morning that we are unable to login to our production instance - cannot state how long this problem already exists exactly. When providing correct username and password, the UI will redirect and throw an "Your authorization token has expired" error in a modal popup. Only thing that is possible in this situation is to click "Login" which will then redirect on another page which then will redirect again. This redirect loop will again result in the same error popup. We already tried removing all cookies/local storage from the browser, tried all other browsers.

Is this a known ongoing issue? We havent changed anything on the instance in the last weeks? We are running on cloud version v4.6.1.

Cheers,

Stephan