Feeds suddenly does not update

One second the XML feed worked. Another second it no longer updates my feed.

The feed is quiet simple:

<items>
  <item>
    <GUID>67e16370-6040-11dc-b343-000ea68e967b</GUID>
    <PUREDATE>2007-09-11T10:24:25.131+02:00</PUREDATE>
    <LANGUAGE>Engelsk</LANGUAGE>
    <TITLE>Variation in Genetic Markers</TITLE>
    <DATE_YEAR>2007</DATE_YEAR>
    <AUTHORS>John Doe</AUTHORS>
    <AUTHORS>Lisbeth Mikkelsen</AUTHORS>
    <AUTHORS>Niels Roulund</AUTHORS>
  </item>
  <item>
    <GUID>034883a0-b0fd-11db-bd61-000ea68e967b</GUID>
    <PUREDATE>2007-01-27T07:47:56.920+01:00</PUREDATE>
    <LANGUAGE>Dansk</LANGUAGE>
    <TITLE>Mykorrhiza - en naturlig del af en græsrod</TITLE>
    <DATE_YEAR>2006</DATE_YEAR>
    <AUTHORS>Sabrina Doe</AUTHORS>
  </item>

However, all of a sudden the feed is not updated anymore and displays the message

There are no new content.

In my effort I tried to delete all my feed content hoping that this would make it realize that there was no corresponding GUID record, and thus would add the content once again. However, nothing happens...

Suggestions are more than welcome ...

Additional information (2014-04-07): Try this link for an example feed..