login failed for user '(null)'| trusted connection

I had a report that was working and then this error started coming up whenever I tried to view it in the preview window.

login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection

When I upload the report rdl to our server and use an established data source the report works fine.
I have two questions. 1) Why would this error occur and 2) how do I fix it?

My guess is that I have to restablish a conenction to a working datasource but I don't know how to do it.

Any help would be greatly appreciated.

Thanks.

Is your datasource set to use integrated security? If so then you are probably hitting the double-hop problem.

http://blogs.msdn.com/jgalla/archive/2006/03/16/553314.aspx

|||I'm looking into this now but as some reports work and I can remake this report so that it functions I think it might have to be the report itself.

This report uses a private data source and I suspect that there might be a problem with it. How can I change a private data source without editing the RDL manually?

Again thank you for the help.
|||I suspect that the reports that are functioning are using different credential options. You can view how the data sources are configured in Report Manager from the "Data Sources" tab for each report.|||I've received the same error but under different circumstances :
Login failed for user ". The user is not associated with trusted SQL Server connection.
The datasource is connecting to the database using SQL Server Authentication. First time round this report previews fine. I then copy and paste the report to obtain an identical one (with the name of "Copy of report1") and then try and preview this new report, but get the above error. The original report still works fine. In the copy, all the dataset queries work fine when run individually, the error only comes when i try and preview the report. Any ideas why this is happening?
Guy|||Further to the previous post...
I later closed the original report and reopened it to find the original gave the same error as above. After much investagation it appeared that the datasets had somehow changed there datasources. They were all pointing at datasources with the correct names, but not shared ones - i dont recall setting up any that were not shared, and they certainly are not showing in the solution explorer.
On re-pointing the sets to the correct shared sources it now all works fine and copy and pastes fine too. But why should they have changed during a copy and paste and even during a save and reopen? ?sql