How much free space is there for the SFTP server to use? If you can’t assess, you could probably upload some 50 MB file (whose name doesn’t start with duplicati- to avoid confusion) to test it.
50 MB would be the default Remote volume size on Options screen. Are you using that?
Duplicati freezes during backup or fails with unknown error due to Remote (SSH) Disk Full #4675
Renci.SshNet.Common.SshException: Failure
at Renci.SshNet.Sftp.SftpSession.RequestOpen
at Renci.SshNet.SftpClient.InternalUploadFile
(simplified, but looks like the end of both of yours)
That’s probably just testing that a file listing doesn’t error. You might have an uploading problem.
Your list of cleanup attempts is also talking about a lot of files that tried uploads, but aren’t there.
Although any SFTP client should be able to test uploads, Duplicati has two testers you could try:
BackendTool is run manually. BackendTester does an automated test that uses an empty folder.
Export As Command-line can get a URL to start the test with your server.