There is a space missing on collapse_api.php on line 111 which results in the following error:
XML Parsing Error: not well-formed
Location: https://mantis.perot.nl/mantis/bug_report_page.php
Line Number 112, Column 34:
In firefox 4.0pre8.

fix:
111c111
< echo '<div id="', $t_div_id, '"', ( $t_display ? 'class="collapse-open"' : ' class="collapse-closed"' ), '>';

  echo '<div id="', $t_div_id, '"', ( $t_display ? ' class="collapse-open"' : ' class="collapse-closed"' ), '>';