How to Set Up WinSSHD SFTP Server On Windows in on Bitvise server. May suit you if free personal license fits. I think I might have tried it once for SFTP, but it also does FTPS, so post may need tweaks.
Other alternatives may be here. Sorry I haven’t personally used FTPS (setting up SFTP seems easier):
Fatal error: Detected non-empty blocksets with no associated blocks launched an effort to try to find how that problem can be reproduced. While I’d like for you to not get it, if you know how to get it reliably, would you be willing to join the chase on some test backup on FileZilla Server, which also keeps a thorough log?
What Duplicati version is this, and is it “FTP” or “FTP (Alternative)”? (and you could also test both flavors)