Synchonisation via WebDav fails after a while

I synchonize Joplin in following devices:
iPhone iOS 14.2, Joplin 10.4.1
iPad iOS 13.7, Joplin 10.4.1
Synchonization Target: WebDAV (Wed.DE, german provider)

After a while (several days synchronizing properly), Joplin on the iPad does not synchronize any more. Instead of the button for synchronization, a button "Abbrechen" (German for interrupt, abort) appears, the arrows circle keeps circling. The iPhone continues synchronizing properly.

Remark.: I edit new notes mainly on the iPad.

I included a log but do not know if the PDF-File is included: