Sql Server 2008 Reporting Services: Saved report in report builder 2, now cannot open. Says: Untitled.rdl, Deserialization failed

I have recently installed SQL Server on a Windows 2008 (64) bit
Installed Reporting Services.
Jobs to copy Production SQL DB to this server runs every night.
Everything works fine.

I have upgraded several reports from older report servers (sql 2000 & 2005)  No problem

2 or 3  times a report suddenly cant load in report builder. It is a report which i have been working on, and uploaded to Report Server website, no problem. Then without warning, a report suddenly cannot be loaded.
Even though it works perfectly in the website, i cannot load it in report builder.
it cannot be loaded from disk (.rdl) either. Even though the .rdl can be uploaded to the reportserver website (web) and works here.

In previous cases i just restarted with old version, but this is 4 days work wasted, if I cannot continue.

When i load file (or web) in report builder i get message: "Failed to open report 'Untitled.rdl'" Details: "Attribute 'DataType' not declared. Deserialization failed: Attribute 'DataType' not declared. Line 6844 position 20' (translated from danish)

I then get a new empty report instead of the 1 I tried to load.
If I load the file into an editor, I get 

Line 6831
    </ReportParameter>
    <ReportParameter Name="numberPrice">
      <DataType>Integer</DataType>
      <DefaultValue>
        <Values>
          <Value>1</Value>
        </Values>
      </DefaultValue>
      <AllowBlank>true</AllowBlank>
      <Prompt>Antal og Pris</Prompt>
      <ValidValues>
        <ParameterValues>
          <ParameterValue>
            <Value DataType="Integer">1</Value>
            <Label>Antal og pris</Label>
          </ParameterValue>
          <ParameterValue>
            <Value DataType="Integer">2</Value>
            <Label>Antal</Label>
          </ParameterValue>
        </ParameterValues>
      </ValidValues>
Line 6853

I have no Idea what is wrong with the line: 6844  -  "<Value DataType="Integer">1</Value>"
Or if Report builder 2 has an error.


Please advise   :-)

Mikaelvv