Yes, as long as the SR is not used by any other host/pool, it's safe to remove it. I'm talking about the SR NFS folder created by XCP-ng. Don't touch anything inside the folder except for entire removal of it.
@olivierlambert Without any change in the configuration, the backup now reports to be successful. Next step is to check whether the backups will restore correctly.
The mystery still stands - what was wrong previously.