Thats just the custom error page we show so that hackers don't get to see any error detail.
Lets turn off that feature so you can see the error and then when its resolved you can enable it again.
Look for this in Web.config:
<customErrors mode="RemoteOnly" defaultRedirect="Error.htm">
<error statusCode="404" redirect="~/PageNotFound.aspx" />
Change "RemoteOnly" to "Off" and this will allow you to see the real error that is causing the problem.
Hope it helps,