The usual suspect is mootools being loaded twice, however after commenting out line 245:
<script language="JavaScript" src="<?php echo $mosConfig_live_site."/modules/miniadspot_js/mootools.v1.11.js";?>">
...the problem still exists. So, the next step is naturally to comment out line 244:
<script language="JavaScript" src="<?php echo $mosConfig_live_site."/modules/miniadspot_js/wz_tooltip.js";?>">
and now IE loads fine. Only problem, that gets rid of the tooltips.
Solution --> is there a way to use JQuery or something else to load the tooltips? A module parameter to choose between mootools or jquery would be awesome (or something besides jquery).