When creating CIFS mounts, Webmin uses the
\\servername\sharename syntax. While this is basically correct, and the usual notation under Windows, there is an issue with that under Ubuntu.
A CIFS mount with a subdirectory, like
\\server\share\path, fails to mount at boot time, with some "syntax error in share name: does not start with \ or \" error. Manually mounting it subsequently in a shell does work.
//server/share/path instead, it works correctly also at boot time, also without "path".
So, while I'm not sure whether this is an Ubuntu issue, maybe it'd be an idea to have Webmin use forward slashes in this case.
On another note, Webmin incorrectly splits the mount target into the edit boxes "Server name" and "Share name":
\\server\share ends up in "Server name", and
path ends up in "Share name".
I suppose the developers did not reckon so far that some weird guys want to use CIFS mounts with path names behind the share. ;)