I found that virtualmin 1.500 deletes /etc/apache2/available-sites/default , leaving the symbolic link to the sites-enabled/000-default (Debian 5.0) when disable ssl vsite feature, and left the ssl configurations there...
This prevents apache2 from restart. I had to manually create a default file, as virtualmin seems to not save the file again (maybe because it sees the symbolic link at sites-enabled directory).
I hope this report helps to improve such good tool.
Andre Felipe Machado