‘No files were found at the remote location’ using Onedrive for business
Failed Backup with id: 3 (Missing Files)
suggest that the Storage Providers listed as using “Microsoft Graph API” can go past the old 5000 limit.
I have no idea at what point (if ever) the Microsoft web UI gets cranky about having too many files to list.
If the backup is not running yet, I’d suggest setting blocksize to 1 MB. 100 KB is small for a big backup. Alternatively perhaps several smaller backups could be used – and could even go into different folders.