Is it possible to use:

crvViewer.ReportSource = MapPath("Preview\August.rpt")

You would have to have the "Preview" directory located inside the folder where the web application resides, but that could be a possible solution.

Also, another point to consider... ASP.net relies on the security permissions of the ASPNET user on the machine where the site are hosted. If the ASPNET user (on the hosting machine) does not have READ / WRITE security permissions to the directory where the report files are located, that could give you a lot of problems. I've found that it's good to give the ASPNET user READ / WRITE permissions to the TEMP directory as well.

These are just some starting ideas. I went through all kinds of hoops trying to get CR to work well.