Menu always opened, mouseover not working, no error message

[rbachl]

I cannot get the udm4 menus working and would need some help.
The menu is permanently displayed, mouseover is not working, but there is no error message.
When looking at the source code generated by the web server, everything seems to be ok. In particular, the following lines are included:

(----snippet 1-----)
<link rel="stylesheet" type="text/css" media="screen,projection" href="udm/udm-style.php" />

(----snippet 2-----)
<ul id="udm" class="udm" ><li><a href="./index.php?ee_lang=eng&amp;u=">Browse&nbsp;Gallerie s&nbsp;</a><ul><li><a href="list.php?exhibition=1&amp;u=&amp;ee_lang=eng " title="1 photo">First exhibition</a></li><li><a href="./" title="--------------------------------">--------------------------------</a></li><li><a href="./guestbook.php?u=&amp;ee_lang=eng" title="Guestbook">Guestbook</a></li><li><a href="./news.php?u=&amp;ee_lang=eng" title="News">News</a></li><li><a href="./list.php?exhibition=all&amp;select_output=ratings& amp;u=&amp;ee_lang=eng" title="Browse Ratings">Browse Ratings</a></li><li><a href="./sitemap.php?u=&amp;ee_lang=eng" title="Gallery Map">Gallery Map</a></li><li><a href="./gear.php?u=&amp;ee_lang=eng" title="Gear List">Gear List</a></li><li><a href="./comments.php?u=&amp;ee_lang=eng" title="Photo Comment Browser">Photo Comment Browser</a></li></ul></li></ul>

(----snippet 3-----)
<script type="text/javascript" src="udm/udm-dom.php"></script>

udm-dom.php, udm-style.php and udm-custom.ini are all readable in subdirectory udm. The paths to udm-custom.ini are set correctly in the files udm-dom.php and udm-styles.php.

The php and apache versions are a bit dated, but I have not encountered any other problems with this before. I was following the description for integrating udm in Exhibit Engine as described in http://photography-on-the.net/forum/...d.php?t=223312
which includes all other details of the menu configuration.

Do you have any suggestion what I could modify or check? Thank you.