After working for some weeks without problems, the following issue occured from one day to another and is now reported every day by email:
Doing apt-get update and apt-get-upgrade manually by SSH works without errors. I'm not completely sure, but the issue could have occured with the...
Clear your browsers Cookies and it will work again.
See my post in this thread:
https://support.plesk.com/hc/en-us/articles/360011461379--BUG-Advanced-Monitoring-showing-login-OAuthLogin-missing-saved-state-