try with this:
RewriteEngine On
RewriteBase /
RewriteRule ^forum/([0-9]+)$ /modules/newbb/viewforum.php?forum=$1 [L]
And try 'www.mydomain.com/forum/1' or what the forum number might be.
Did you remember this part:
I also had to change 1 line in includes/common.php
from:
$xoopsRequestUri = $_SERVER[ 'REQUEST_URI' ]; // Deprecated (use the corrected $_SERVER variable now)
to:
$xoopsRequestUri = $_SERVER[ 'SCRIPT_NAME' ];
If i remember correctly you will actually get blank screen if you forgot it.
I have no clue why it is not done in the core system allready, they even themself write it is 'deprecated'.