After checking the generated source I found

Code:
<a href="#" onclick="haalnieuwslijst('-datum','desc','1','');return false;" class="linkmethokje">1</a>
<a href="#" onclick="haalnieuwslijst('-datum','desc','2','');return false;" class="linkmethokjesel">2</a>
<a href="#" onclick="haalnieuwslijst('-datum','desc','3','');return false;" class="linkmethokje">3</a>
<a href="#" onclick="haalnieuwslijst('-datum','desc','4','');return false;" class="linkmethokje">4</a>
<a href="#" onclick="haalnieuwslijst('-datum','desc','5','');return false;" class="linkmethokje">5</a>
where -datum should be datum without the -
did a str_replace for the - and it was fixed.