Thanks for the advice, but I tried it before and again, w/o any changes.
The same applies by opening mail on icloud in safari, chrome etc. in os or win.
After some seconds an error message appears including some details like
...
REPORTED ERROR TYPE
CANNOT_LOAD_STARTUP_DATA
...
WARN: REJECTING SERVER RESPONSE CoreMail.MailRequest.willReceive:
...
ERROR: Bootstrap error: FolderList.LoadFailed.ServerError
...
I called today with the Apple support but unfortunately several agents had no solution which works. They hope, that there are maybe some migration processes running which lead to mentioned issues and want to call me tomorrow to see if there are some changes. ;-(