3
In include/common.php there is a line that looks like this:
$url_arr = explode('/', str_replace(str_replace('https://', 'http://', XOOPS_URL.'/modules/'), '', 'http://'.$HTTP_SERVER_VARS['HTTP_HOST'].$xoopsRequestUri));
This line fails to do the correct str_replace if you are using a virtual server or have named your server in apache.conf something other than the name of the servers hostname. You can replace HTTP_HOST with SERVER_NAME and it will fix this problem. This may or may not be what you are having problems with, but I had the same problem and this fixed it.