Ahh ok, I've never connected to excel with SSRS but in a previous job we did have problems with the SSRS user not having access to network drives when it came to publishing reports to a file location, we set up a windows user id dedicated to SSRS which had network rights and it worked. Probably not the most secure method but if it was only a system in development at the time.

In saying that though, you say it returns data in the query designer, but not the preview, don't they both use the same connection string?

Post back with an update if it works or not 🙂