Hi Lars,
It shows a 500 error but it is not clear if mojoportal is handling the request. I don't suspect a Web.config error if all other urls in mojoPortal work.
What I would try is first clear the mojoportal log then cause the error and see if anything is logged or not.
If not, then maybe try disable the friendly error as described in Basic Troubleshooting and see if you can get a more detailed error.
I would check in the web control panel that Default.aspx is the default document and not something else like index.aspx or index.php etc.
Hope it helps,
Joe