Ooops, it happened again.
I got expiration notice mails from Letsencrypt for two domains.
I inspected it showing same message "Incomplete authorizations"!
It stayed for several days, so i suspect i-mscp plugin is not handling further LE updates!
After I pressed the Button "Status von diesem Posten für einen neuen Versuch ändern" the message switched to OK.
In the moment, everything is clean. Stay tuned...