Error The base connection is closed (Webdav Yandex) is another report of Yandex WebDAV stopping working with Duplicati, so you’re not alone. I don’t know what they changed, or if they’re trying hard to push users onto Yandex Disk which appears to be a paid service. It looks like it has a REST API with OAuth authentication, but Duplicati OAuth Handler is not set up, thus it’s WebDAV or nothing. Sorry…
Debugging WebDAV might also be possible by somebody (probably not me), but encrypted protocols impede protocol viewing, so possibly it would have to be done by somebody using a debugger on C#.
Possibly you can get Duplicati to write out the decrypted version of some Yandex error message from WebDAV. This may be more likely from command line tools Duplicati.CommandLine.BackendTool.exe
or Duplicati.CommandLine.BackendTester.exe though possibly you can spot something using the GUI live log at About --> Show log --> Live --> Retry. You could also have a log file at some level, e.g. retry.
First simple thing to test would be the “Test connection” button on the Destination screen for small test which just tries a file listing. If that doesn’t work, something’s way off, and other things will probably fail.
Found 3 remote files is a new issue seen on Yandex.Disk presumably on WebDAV, but I’ll ask about it.
I’ll probably point back here for troubleshooting, so if anyone has anything to add from this, please say.
Oddly, the new report appears to be about extra files, whereas the issue here seems to be lack of files.