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.