Turn On Remote Error Reporting
If remote error messages are not enabled, SSRS will not display the actual reason behind an error message for security reasons.
In order to troubleshoot, however, you need to know what is happening, so watch this video to see how easy it is to enable remote errors.
Once you turn on remote error reporting you will be able to see the actual reason behind any error message.
Complete step-by-step instructions can be found in this blog post.