Duplicati.CommandLine.BackendTool.exe let me try out variations (and see errors) a bit more easily, and the suspect wound up being Percent-encoding in a URI which I’m not sure is being done correctly. You can follow that chart, converting a plus sign to the sequence %2b or %2B to see if it lets plus work. That’s a workaround, not a fix. For a code fix (if manual encoding actually helps), please file an issue to get the fix request queued.
This same issue possibly explains this problem found by @thepappaq where workaround was to do a folder rename. That one was a little murkier because the # (number sign or pound sign) is also an invalid character sometimes, so Microsoft Graph’s unifying things could maybe have unified restrictions, but plus is not invalid:
Invalid file names and file types in OneDrive, OneDrive for Business, and SharePoint
Thanks for testing out variations of names. If you would like, feel free to try other characters that should also be percent-encoded. The test data for the URL encoding theory so far is a little thin now, but could be better.