I recently started using the Backup Virtual Servers option with Virtualmin 2.601 (GPL version), and I assume this problem still exists in the Pro EA1 version (but have not tested it yet). It appears that this function backs up selected or all domains to /tmp before writing the information to the selected destination. However, /tmp is usually not that large, especially not if backing up all files in all domains at once, so /tmp quickly fills up (which wreaks havoc with other system processes), and the backup ultimately fails with this error:
Backup failed : Failed to write to /tmp/.webmin/{PID}_1_backup.cgi/{DOMAIN}_{FILE} when closing : No space left on device
Can these files be backed up directly to the selected destination rather than dumping them into /tmp first?