Usually if you can't even get an error detail it means it doesn't like the Web.config file. This is a common problem if the framework versions are not correct like if you installed the mojoPortal .NET 4 package under .NET 3.5. You did not mention what package of mojoPortal you used.
In this case you should be able to find the error in the Windows Event Viewer in the application log.
Also make sure you have SP1 installed for .NET 3.5.
Hope that helps,
Joe