OK, I found it. But I'm not sure if this is really a solution or if I mess something else up.
In global.php find this
$url = preg_replace(array("/[?|&]action=[^?|&]*/", "/[?|&]mode=[^?|&]*/", "/[?|&]".URL_ID."=[^?|&]*/", "/[?|&]l=[^?|&]*/", "/[&?]+$/"), array("", "", "", "", ""), $url);
and replace with this
$url = preg_replace(array("/[?|&]action=[^?|&]*/", "/[?|&]".URL_ID."=[^?|&]*/", "/[?|&]l=[^?|&]*/", "/[&?]+$/"), array("", "", "", ""), $url);
As you easily spot the "action", "mode" and a few other things are removed with this regex. I assume that there was a reason for doing so but I don't what it might be. Anyone have an idea?
cheers,
b.