During a recent process of restoring a site from a Virtualmin backup archive I encountered this discouraging error message during the upload:
Restore failed : The specified source does not contain any domains, or is not a Virtualmin backup
No need to say that the archive indeed contained several domains plus it was created by Virtualmin itself in the past…
A quick search provided me with a hint that the upload and restore process may not work as desired, but instead first upload and thereafter restore the backup archive from the local server may give much better results.
So simple that I should have tried it the first.