Check this forum entry for bug details:
http://moodle.org/mod/forum/discuss.php?d=144328

look for the line:

this.resources_ul = this.content_td.getElementsByTagName('ul')[0];

in /lib/ajax/section_classes.js (line 143 on my version)

and replace it with this code:

this.resources_ul = YAHOO.util.Dom.getElementsByClassName('section','ul',this.content_td)[0];