Thanks for all the suggestions.
Finally I got it working. Since I started working on it, a new version of tiny has become availabe, I did the update to 2.9.1 and now it works for me...
But there's still a problem left: I am using colorbox to display the form to add a comment and tiny doesn't work in it. Tiny works fine if I don't use colorbox, and colorbox works fine if I don't use tiny

In the attached image file you can see what it looks like: the editor appears, but the text area has the background color and it is impossible to write in it and to click the icons on the toolbar.
This is where colorbox is defined in the head:
<__script__ type="text/javascript">
$(document).ready(function(){
$(".kommentar").colorbox({width:"750", height:"540", inline:true, href:"#kommentarbox"});
});
</__script>
And this is where it is finally called and the form itself:
<div id="kommentarbox" style="padding: 5px; background: #ffff00;">
<h1>Eppes an d'Gëlle Buch androen</h1>
<br /> <div class="cgfeedback_addcoment">
<form id="m74e64moduleform_2" method="post" action="
http://www.groupe-animateur.lu/index.ph ... gelle-buch" class="cms_form">
<div class="hidden">
<input type="hidden" name="mact" value="CGFeedback,m74e64,default,0" />
<input type="hidden" name="m74e64returnid" value="3" />
<input type="hidden" name="m74e64key1" value="Gëlle Buch" />
<input type="hidden" name="m74e64key2" value="x" />
<input type="hidden" name="m74e64key3" value="Groupe Animateur" />
<input type="hidden" name="m74e64feedback_origurl" value="
http://www.groupe-animateur.lu/index.ph ... gelle-buch" />
</div>
<table>
<tr>
<td align="right">Ären Numm: </td><td align="left"><input type="text" name="m74e64author_name" size="30" maxlength="255" value=""/></td>
</tr>
<tr>
<td align="right">Är Emailadress:</td><td align="left"><input type="text" name="m74e64author_email" size="30" maxlength="255" value=""/></td>
</tr>
<tr>
<td align="right">Titel vun Ärem Kommentar:</td><td align="left"><input type="text" name="m74e64title" size="40" maxlength="255" value=""/></td>
</tr>
</table>
<table>
<tr>
<td align="left">Äre Kommentar:</td>
</tr>
<tr>
<td><textarea name="m74e64comment" cols="80" rows="15" class="cms_textarea"></textarea></td>
</tr>
</table>
<br>
<table>
<tr><td align="center"><input type="hidden" name="hncaptcha" value="11332323133331113333233133132312"/>
<input type="hidden" name="hncaptcha_public_key" value="65ce3"/>
<img class="captchapict" src="/tmp/cache/hn_captcha_65ce3.jpg" width="198" height="72" alt="This is a captcha-picture. It is used to prevent mass-access by robots. (see:
http://www.captcha.net)" title="">
</td><td></td></tr>
<tr><td align="left">
Gitt w.egl. d'Zeechen an, déi dir am Feld hei uewendriwwer gesitt:
</td>
<td>
<input type="text" name="m74e64feedback_captcha" value="" size="20"/>
</td>
</tr>
</table>
<br>
<input type="submit" name="m74e64submit" value="Androen"/ >
</form>
</div>
</div>
And here you can see it in action:
http://www.groupe-animateur.lu/index.ph ... gelle-buch
For those few people who don't speak luxembourgish, the first link calls the form

. At the moment the wysiwyg editor is deactivated, therefore it works.
So I would be very happy if anybody had an idea how to fix this or could tell me if this is impossible with colorbox...
Thank you very much!