“No disks have been mounted. Either disks of this type aren’t supported or some errors occurred. See log file for details.”
To view the log files browse to C:ProgramDataVeeamBackup, and look for the log file Veeam.File.Explorer_date_time.log, there are a lot of files here so a quick sort by date is useful.
It looks like it is a known issue since version 5.x, and the resolution is to reboot the Veeam server. This to me seems a bit inconvenient especially if you are doing a disk to disk to tape and the need to restore a file before the tape job finishes.
The Veeam kb article can be found here.