Dear Garima, I am happy to inform you that the sync works again on all of my devices!
As you suggested, I disconnected all my devices and ticked the option to delete the information from WebDAV as well. After that there still remained a (smaller) file called 'vault.enpassdbsync' in the Enpass cloud folder on WebDAV, which I deleted as well. Then, I linked my main device to WebDAV again, and after that, the sync was functional again.
I suspect the problem might have occurred because of my slow internet upload speed, which might have caused the sync file to get corrupted.
Thanks a lot for your help!