Pretty obvious that DevonThink is using CloudKit API calls and doing it correctly as it worked fine for me for years and years.
Also pretty obvious that an HTTP 500 error means that the server endpoint (ie Apple’s dominion) crapped out. We don’t know why though - only Apple knows. I know from having enough of my own server architecture that it’s likely to be 1) resources overdrawn (load balancer can’t get anything live backend, etc); 2) some sort of privilege issue (meaning Apple has some bad plist file somewhere probably); 3) some sort of bad workers/event threads (failure to garbage collect or recycle children); or 4) and MOST LIKELY for Apple - cache failures of some kind in the middleware.
This is what I was getting at in my other thread reply where I mentioned that verifying the iCloud database still works as well as verifying the location.
Just a guess on my part but likely Apple has some middleware between the endpoint and storage and it or the pipes are ganked until some garbage collection or process recycling happens.