deleted a calendar but em-client still tries to synchronize via caldav

Hi,
I have recently deleted a calendar which synchronized via CalDav. Now em-client still tries to synchronize it and gives an error message
The synchronization of … has failed. XXXX@mailbox.org (not found)
But mail and other calendars (in my mailbox.org account synchronize at the same time without problems.
I use Win10 and the latest em-client.

Thanks for any help or hint.