I am having the same type of problem. My last successful sync was on 11/26/21. Interesting that @Andreas76 reported this issue around the same time.
The sync hangs for long periods of time. I then receive the error message “Request failed with http status code 503,” and “CKErrorDomain 6” in the info panel. I am also using iCloud (CloudKit).
I have read through and tested the various fixes mentioned in the long thread referenced by @Andreas76.
None of these fixes worked. I don’t think the issue is with a rate-limit on my uploads anyway. My databases are still relatively small (<1 GB), and they were fully synced and working successfully for several weeks before this issue appeared.
To simplify troubleshooting, I have turned off sync for all databases except for the Global Inbox. I have verified, repaired, cleaned, restarted, rebooted, etc. Each individual step reports as successful, but the sync continues to fail and show the 503 error.
I am also running DTTG on my iPhone. This may be relevant, as the items I have added to my Global Inbox for DTTG have also not synced since around the same date. But I really have no idea whether the problem originated on DT3 or DTTG.
I am not aware of anything unusual about any of my recent files; i.e. none are particularly large or added via a method different from anything I have successfully done in the past.
Computer info:
MacBook Pro (16-inch, 2021; M1 Pro)
macOS Monterey v12.0.1
DT v3.8
Phone info:
iPhone 13 mini
iOS v 15.1.1
DTTG v3.2.4 (17157)
Any help you are able to provide would be greatly appreciated!