Was this the one-and-only backup ever done? If not, some time and files are probably from another run.
"BackendStatistics" in Complete log will show you things like file uploads and deletions for the run.
“Could not find file" naming a temporary folder file is an unsolved mystery at backup’s last data uploads, possibly having something to do with an early error. Can you see “SpillCollectorProcess” in stack trace? That’s what picks up partially filled temp files left over from when the earlier parallel file-fillers were done.
was a similar problem on OneDrive, and I suspect timeout needs to get longer. Possibly yours does too, however so far the evidence hasn’t been posted. You might also spot “RetryAttempts” in the backup log.
You can drag-and-drop files into the forum if you like. If it doesn’t like the file type or size, it’ll let you know.
Or you can paste links to files. Or using three backticks (```) above and below creates a scrolling viewer.
I’m not sure logs will answer all the questions, but they might help. Creating a bug report is another path.
The “Unexpected difference” error is from an internal consistency check on the database before backup.