Following the only clue provided, the ReadDoneAsync2 looks like Duplicati asked mono to do a WebDAV operation, and Yandex sent nothing (zero bytes), or possibly closed the connection at:
To actually see if a connection closed at the network level is possible if you’re willing to capture packets with tcpdump, wireshark, tshark, or similar. Encryption gets in the way of doing a good higher-level look.
You can possibly get a Duplicati-level look by looking either at About → Show log → Live → Retry or <backup name> → Show log → Remote and click on the downloads and uploads around time of error. Possibly you can find a multiple-line stack trace which would say more about where Duplicati was then.
So to confirm, you’re saying you see new files showing up at Yandex, viewed somehow separately from Duplicati, and at some point things stop working? What does the status bar say at the time of the error?
You can also watch live log as above. The backup should do lots of uploads, but at the end it will switch to Verifying backend files or Compacting files at the backend. When do you see that the error happens?
Have you used the “Test connection” button on the Destination screen a few times? Does it run reliably? This is generally just a directory listing, so it might work even if uploads or downloads are having issues. Does the <backup name> → Verify files button work well? That’s similar to the verification after backup.
In looking over past Yandex reports, I see reports in 2017 and 2018 that it worked, but recently it hasn’t. How’s your experience been on other systems (if you run any)? You might also have a system problem.
You can run an Internet speed test to see if your network connectivity seems to be doing what it should. Testing Yandex specifically can be done with Duplicati.CommandLine.BackendTester.exe pointing to an empty folder. The URL to give that command can be based on the URL from Export As Command-line.
For whatever it’s worth, below are other recent poor Yandex experiences, so issue possibly is their end.
Error The base connection is closed (Webdav Yandex)
Duplicati cannot finish archiving
EDIT:
Has stopped working Duplicati 10 errors constantly
At least some of the above look like Canary versions (and yours is). I wonder if older Beta would work, although it’s got a lot of other problems? If you can test 2.0.4.5 or 2.0.4.23 to Yandex it may be helpful.
For a real guess on Canary, you could also try reducing --asynchronous-concurrent-upload-limit to 1 in case somehow the default 4 concurrent uploads causes an issue. Beta doesn’t do concurrent uploads.