Non consistant behabviour

I've found a strange behavior of the virtualmin server:

I've installed 3 websites. all are php based. the 1st two worked great from the start. The 3rd one, which is the same as the previous, got an error accessing the domain.com path - no permissions.

the only way to resolve it was to add an .htaccess file to the directory of web site to cause the server to serve default.php

this file was not needed in the 2 other sites.

what happened ?