The report might want to access a DataSource or DataView where the AD user (or AD group) has insuficcient access rights.
Make sure you check out the following URLs:
http://REPORTSERVERNAME/Reports/Pages/Folder.aspx?ItemPath=%2fDataSources
http://REPORTSERVERNAME/Reports/Pages/Folder.aspx?ItemPath=%2fDataSets
Then choose Folder Settings
(or the appropriate individual DataSource
or DataSet
) and select Security
. The user group needs to have the Browser
permission.