On iOS with iPhone Xs running iOS 12.3.1 and Devonthink To Go 2.7.1. I started the database on my iMac. Sync to my iPad fine. My iCloud storage has more than 1TB avail and my iPhone has more free space than the iPad - about 185GB free.
So is there something else that can cause this error? I couldn’t see how to troubleshoot so any suggestions are welcome. Two other smaller databases synced to the iPhone without a problem.