@andrewreid Not that this helps with your error, but my nightly S3 delta backups to Wasabi have been working just fine. I'm using XO source and keep mostly up to date with master.
S3 and NFS are different backup formats so NFS working does not mean S3 will work.
Sometimes Wasabi causes failures but not recently and not that last more than a few VMs in one night. The next backup run (manual restart or next nightly) seems to work correctly.
I force a full backup every 3 months just to make sure I have a good checkpoint in case undetected corruption creeps into the delta data. Plus I have replication and other backups.
I did have a problem once (not exactly yours) with a VM and just deleted the S3 backup data which forced a full backup again. Sucks because you don't have the weeks of deltas anymore.
Error #3 (date-time) is strange. May be a backblaze issue.
You could create a new S3 remote to backblaze (new bucket or directory) and test one failing VM backup to the new remote and see if it works (and works again for delta). You could set the retention low to test merge too. I have a setup like this with a local MinIO server for S3 testing and local backups.