[EventCalendar] XML returned twice in IE (6&7)

[deliciousdays]

Hello everyone!

I'm at a loss. I've tried turning off plugins one by one, tracing the 
HTTP etc. etc. but can't understand why Firefox (2x) is handling this 
URL (http://www.vinoroma.com/?ec3_xml=2007_10) just fine, the source 
HTML source shows:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><calendar><month id='ec3_2007_10'>
<day id='ec3_2007_10_25' is_event='1' titles='Merlot Weinprobe2' link='http://www.vinoroma.com/?m=20071025&amp;cat=6'/>
<day id='ec3_2007_10_30' is_event='1' titles='Merlot Weinprobe2' link='http://www.vinoroma.com/?m=20071030&amp;cat=6'/>
<day id='ec3_2007_10_31' is_event='1' titles='Merlot Weinprobe2' link='http://www.vinoroma.com/?m=20071031&amp;cat=6'/>
<day id='ec3_2007_10_24' is_event='1' titles='Cabernet Weinprobe mit längerem text' link='http://www.vinoroma.com/?m=20071024&amp;cat=6'/>
</month></calendar>

while IE does this:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><calendar><month id='ec3_2007_10'>
<day id='ec3_2007_10_25' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071025&amp;cat=6'/>
<day id='ec3_2007_10_30' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071030&amp;cat=6'/>
<day id='ec3_2007_10_31' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071031&amp;cat=6'/>
<day id='ec3_2007_10_24' is_event='1' titles='Cabernet Tasting Event' link='http://www.vinoroma.com/?m=20071024&amp;cat=6'/>
</month></calendar>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><calendar><month id='ec3_2007_10'>
<day id='ec3_2007_10_25' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071025&amp;cat=6'/>
<day id='ec3_2007_10_30' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071030&amp;cat=6'/>
<day id='ec3_2007_10_31' is_event='1' titles='Merlot Tasting Event2' link='http://www.vinoroma.com/?m=20071031&amp;cat=6'/>
<day id='ec3_2007_10_24' is_event='1' titles='Cabernet Tasting Event' link='http://www.vinoroma.com/?m=20071024&amp;cat=6'/>
</month></calendar>

...of course creating a JS error.

Any idea what's going on?

Thank you!
Oliver

Wordpress: WP 2.3;  Event Calendar: 3.1.1._rc1 (upgraded manually to 
work with 2.3)