Hi Jan,
In order to diagnose it we need to find out what error happened.
Usually if a problem is so bad that it cannot show the error by setting custom errors off, I suspect something corrupt about the Web.config file itself. Perhaps in touching the file something got messed up. I have seen cases where xml files and config files can get corrupted during ftp.
If you have direct access on the machine please check the application log for error details.
You may also check the mojoportal log by downloading the file /Data/currentlog.config
Newest errors will be at the bottom. Please post any errors that look relevant but please do not post a giant amount of text from your log.
I would also make sure you have SP1 installed for .NET 3.5.
Hope it helps,
Joe