My first advice would be to use .NET 4 hosting and the .NET 4 package of mojoPortal, I'm pretty sure even GoDaddy offers that.
But all I can tell from what you've said so far is that the error is actually something about web.config file, its failing to load it therefore changing the <customErrors has no effect since its failing to load the file.
This would happen for example if you install the .NET 4 package of mojoPortal under .NET 3.5 or if you install the 3.5 package under .NET 4.
Your host should be able to find out the error detail from windows event viewer, I could only help further with error details.
Best,
Joe