Are you using the recent –usn-policy option? That got some fixes recently but they’re on total path length.
Issue 3311 PathTooLongException when using USNJournal #3456
https://forum.duplicati.com/t/release-2-0-4-3-canary-2018-11-13/5279
PathTooLongException when using USNJournal #3311
Regarding file limits, there aren’t many reports (maybe your invitation will get some) but I did find this one:
Capabilities and limitations? near 1 million files
If you can select shorter pieces out of your error messages, perhaps posting them will get some input too.