CloudKit sync "CKErrorDomain 15"

An http 500 error is a server error, not a DEVONthink or a sync issue. It is typically temporary.

That being said, I would ask if you ​need ​to use a remote sync option like CloudKit. There is a forum post discussing the options and considerations of the available sync methods.: Sync Types Explained​
If you do need a remote option, CloudKit has not been very reliable for many people ​(including developers) ​for several months.​ You may want to consider an alternative.