Some modules (not all, but quite a lot) use a way to access parameters given through the URL or via forms that is being phased out by the PHP developers in PHP 5, but is still usable in PHP 4.
Specifically the modules use $HTTP_GET_VARS and $HTTP_POST_VARS (there are more, but these are the most common) that only work with PHP 5 if a directive called register_long_arrays is set to on in the PHP configuration on the webserver.
So if you run PHP 5 and this directive is not set to on, the modules in question will not receive the parameters and may malfunction, give blank pages or just refresh the page.
The XOOPS core uses the updated way using short parameter arrays ($_GET and $_POST etc.) since 2.0.9