There's a subtle issue that I didn't catch when reading Tim's post.  You can't specify both <ReportServerUrl> and <ReportServerVirtualDirectory>, so if you add a full URL for the <ReportServerUrl> value, you need to delete the <ReportServerVirtualDirectory> value.

Otherwise, this suggestion fixed my problem.  The strange thing is that it had been working for months until it failed today.  Go figure.

Melissa