This may help you where another eM Client user was getting that same error and it ended up being to do with the Caldav / CardDav Delegated eM Client accounts “Sync option”. See the last reply post below from the following thread.
(Post reply fix from @Stefan75 via this thread)
Vielen Dank für die Information, der Fehler war bei CalDAV/CardDav = > Delegierte Konten,… die Synchronisierung weggeklickt und es läuft,… DANKE!!!
(English)
Thanks for the info, the error was with CalDAV/CardDav => Delegated Accounts,…clicked away the sync and it’s working,…THANKS