Somewhat, but with lots of missing functionality, maybe explaining those (501) Not Implemented.
Duplicati default logging is mostly some light logging in job database (which gives an advantage of OS independence). One problem with failed backups is it doesn’t make a job log in the usual job logs, but server log at About → Show log → Stored usually has something. Please look around backup fail time.
Your <job> → Show log → Remote looks awfully insistent on getting a dblock upload. You can click on those to see file hash value. Repeated hash under a different name is likely retry. Usually the limit is 5, however limit is configurable. Under normal conditions, one expects mixed dblock and dindex uploads.
About → Show log → Live → Retry would be enough to see retry, and a Verbose log to logfile will also.
What I’d like to see if it can be seen somehow is R2 detailed explanation behind its Not Implemented, because without that it’s much harder to guess what configuration change might avoid such a situation.
An easy one to try though is changing Client library to use from Amazon AWS SDK to Minio SDK. There are far fewer options in it, but as a shot in the dark, maybe it will just work better than Amazon’s, which does (I think) have some low-level logging in it, but I don’t think anyone has tried running it. See:
Add support for cloudflare R2 #4673
It might also be possible to use Export As Command-line to get the destination URL, then test that with Duplicati.CommandLine.BackendTool.exe and Duplicati.CommandLine.BackendTester.exe to see what operations work (or don’t). Seemingly list is OK, because that’s what Test connection usually does.