Hi again,
here's another update:
I was curious to replicate my idea from last posting. So I deleted sync, set it up, and received error code 908502 again.
Editing entries didn't change the sync behaviour this time.
Creating a new vault, moving all entries to it and setting up sync for it finally worked out, even after deleting and setting it up again.
Then I installed Enpass on a second PC, and successfully restored the recent vault from owncloud.
Klicking the "Sync Now" button afterwards lead to error code 908502 on the second PC.
Sync on the first PC still is running fine.
I'm not sure this will help in any way, I just wanted to let you know.