B2evolution version: 2.4.7

We put this tag in _html_header.inc.php:

<meta http-equiv="Refresh" content="600" />

It works in FF and IE, but not in Opera. After refreshing, instead of our home page we get an apache page with Index of /b2evolution4/skins/evopress, and a listing of the directory. We have tried in b2evolution 4 with the same result (you can put 6 instead of 600 to try).

Any ideas?