SSRS: Enumerate available reports - only works with UseDefaultCredetials

Question

I'm trying to generate a list of available reports from an SSRS 2008 installation, using code as below:

ReportingService2005 rs = new ReportingService2005();
rs.UseDefaultCredentials = true;

CatalogItem[] items = rs.ListChildren("/", true);

foreach(CatalogItem ci in items)
{
   if (ddlReport.Type == ItemTypeEnum.Report)
      ddlReport.Items.Add(ci.Path);
}

This works fine.

If I set UseDefaultCredentials to false and specify rs.Credentials as System.Net.CredentialCache.DefaultCredentials this also works fine.

However, if I set UseDefaultCredentials to false and specify specific credentials I get 401 unauthorized errors back.

The credentials I am trying to use definitely have access to the report server, and I am at a loss to proceed further.