I solved the issue with DEVONthink sync. In my iCloud-Drive settings I disconnect DEVONthink 3 for storing documents in iCloud. I started DEVONthink 3 and get some error-messages that I can't store documents via CloudKit. I closed DEVONthink 3 and connect DEVONthink 3 in my document-section in iCloud. I also set the max connections on 4. Then I sync the db one by one and got no error-messages. Maybe this can help with similar issues in your apps.

Hi, i am using the CloudKit sync with DEVONThink and experience the same errors. I tried everything including changing the number of connections but the problem exist. Syncing is not working. I opened a support ticket at DEVONtechnologies.