Having this exact problem.
My sync target is OneDrive.
Endpoints are Apple devices.
- Mac : Catalina (10.15.7) running Joplin app 1.4.19
- iPhone : iOS 13.3.1 running 10.4.1
Both go into endless sync mode.
iPhone seems to increment number of items to fetch, fetches those, then bumps up. (i.e, fetching "x of 32", then as x gets close to 32, it's "x of 48", then "x of 64", etc)
Mac shows "Last error: Error: Invalid API or resource"
Worked flawlessly until about 2 or 3 hours ago.
Update 1: I tried removing Joplin from phone, rebooting phone, and reinstalling Joplin to phone. Phone starts sync but goes into a never-ending series of "Fetched items" messages as described above. This is new behavior and I've been using Joplin for . . . awhile. Back in the dark times before vim support was in the Mac client. 
Update 2: prior to this problem Joplin has worked flawlessly on phone and laptop with sync target as OneDrive.