This is tested with both 6.5.0 and 6.5.1 dailybuild v2006-12-14 on top of Linux (Mandriva 2006)

Some asx files are not properly parsed and return on the squeezebox the following message
PROBLEM: CAN'T CONNECT TO SERVER FOR:
(URL)

for instance 
PROBLEM: CAN'T CONNECT TO SERVER FOR:
http://media.rnltech.com/wjtlfmlive64.asx

Same message trying using the RadioTime plugin

PROBLEM: CAN'T CONNECT TO SERVER FOR:
http://opml.radiotime.com/Tune.aspx?StationId=30003&title=Live:WJTL&username=marauder

(station WJTL Lancaster, PA)

Several other stations through RadioTime return the same message while others work.

On the above example, pulling the mms URL directly out of the asx file and pasting that into slimserver plays fine, in this case mms://wjtlmedia.rnltech.com/wjtllive64

here are the contents of the ASX file in question...

<asx version = "3.0">
  <entry>
    <ref href = "mms://wjtlmedia.rnltech.com/wjtllive64"/>
  <Title>WJTLFMLive64</Title>
  <Author>WJTL FM Lancaster, PA</Author>
  <Copyright>2006</Copyright>
  <Banner></Banner>
  </entry>
  <Title>WJTLFMLive64</Title>
  <Author>WJTL FM Lancaster, PA</Author>
  <Copyright>2006</Copyright>
  <Banner></Banner>
  <LogURL href = ""/>
</asx>