Developers were able to trace the error. As they found, the problem reason was that backup agent temp files from the previous backup run remained in /usr/local/psa/tmp/ folder:
supervisor.20407
supervisor.11971
agent-runner.13788
agent-runner.1469
agent-runner.19648
It caused agent error in /usr/local/psa/admin/logs/httpd_error.log that file already exists:
Unable to start agent-runner: exceptions.OSError [Errno 17] File exists Traceback (most recent call last):
File "/usr/local/psa/admin/share/agent_runner/agent_runner.py", line 622, in main os.symlink(socket, socket_link)
After these files were deleted several backups were performed one by one without problems.
The reason why previous backup run did not delete temp files in unclear. It does not happen all the times, actually we were not able to repeat situation that temp files remain. Probably it happened because of lack of memory issues which happened several times, UBC showed that privvmpages failcounter was 9 when developers worked there (now VE has been restarted and it is cleared). Please try to keep an eye on this.