Hello Bernd,
thanks for your hint. I did following:
Opened "dark_ritual/info.php" with "Template Edit" in the admin section.
found that
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html>
and replaced it with your code
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
But it seems to have no effect

Seems, like there is a problem with that XML format of the feed and the Newsreader module.
Maybe some strange XML characters, that need to be uncommented in the module to make that stuff IEl compatibel.
Just a idea!
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EDIT
I let the feedvalidator do some checks on the feed and he found some errors:
http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.motorradonline.de%2Frss_nachrichten.xmlDoes that part part cause some trouble?
# <?xml version="1.0" encoding="iso-8859-1"?>
# <rss version="2.0"
# xmlns:content="http://purl.org/rss/1.0/modules/content/"
# >
# <channel>
# <title>MOTORRAD-Nachrichten</title>
# <link>http://www.motorradonline.de</link>
# <description>Akutelle News aus der Motorrad-Szene</description>
# <language>de-de</language>
# <copyright>(Copyright Motor Presse Stuttgart GmbH & Co. KG)</copyright>
# <image>
# <url>http://www.motorradonline.de/favicon.ico</url>
# <title>MOTORRADonline</title>
# <link>http://www.motorradonline.de</link>
# </image>
Here is the first item:
# <item>
# <title>Entschuldigung an Dani Pedrosa</title>
# <description><table align="LEFT" valign="top" cellspacing="0" cellpadding="0"><tr><td rowspan="3"><a href="http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm"><img src="http://images.motorradonline.de/sixcms/media.php/340/thumbnails/0187636.jpg.571263.jpg" title="Dani Pedrosas Vorderreifen hielt nicht so gut durch" border="0" alt="Dani Pedrosas Vorderreifen hielt nicht so gut durch" /></a></td><td rowspan="3">&nbsp;</td>
# <td><a href="http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm"><span style="color:#dc150f;font-size:12px;text-decoration:none">Tohru Ubukata</span></a></td></tr><tr><td><a href="http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm"><span style="color:#000000;font-size: 16px; font-weight: bold;text-decoration:none;">Entschuldigung an Dani Pedrosa</span></a></td></tr><tr><td><span>Es gab viel schweren Regen in Motegi. Was können Sie über die Leistung der Regenreifen sagen?
# Tohru Ubukata: Ich bin recht zufrieden mit der Leistung des Bridgestone-Regenreifens an diesem...</span><a href="http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm"><span style="color:#dc150f;text-decoration:none;"> ... > weiter</span></a></td></tr></table></description>
# <link>http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm</link>
# <author>info@motorradonline.de ()</author>
# <pubDate>Tue, 28 Apr 2009 09:52:00 +0200</pubDate>
# <guid>http://www.motorradonline.de/tohru-ubukata--entschuldigung-an-dani-pedrosa.345959.htm</guid>
# </item>
After that, it continues with the next "<item>" and so on.
Hope someone can help me.
Maybe I try to read that feed with a external PHP-feedreader-page and put it in WB with the inlinewrapper module.
But that would be the dirty way

Perhaps someone can give me another hint ...