I am experiencing this issue also.  I am upgrading from DNN 4.5.3 to 5.2.2, in a development/testing environment to troubleshoot problems prior to upgrading the production server.  The upgrade reports success, but when I open the page, it states "An error has occurred" with no additional information.  I am not getting any entries in the Windows Server 2003 log files either.  If I use a generic index.html file as the home page (setting IIS to use it), that page will load, but as soon as I return to using the default.aspx file, the "An error has occurred" returns.  I am going to start commenting lines in the default.aspx.vb file to see if I can narrow down the potential issue, but this is going to be a slow and tedious process.

We have found the Problem in our Upgrade!

• We have renamed for a test the Directory from the ML-Skin in the Portal-Structure so Dotnetnuke load and use the Default Skin and we see thats works. The Different is we use in our Skin Tokens from Apollo for Multilanguage Menus and other Features.
• We have look at Snowcovered for a Update from Apollos Pagelocalization and found the last with Version 05.00.09.
• We goes back to the DNN Version 05.01.04, install the Update from Apollo, Install the Dotnetnuke Upgrade for DNN 05.02.02 and receive here no Errors and the Portals comes up without any Errors and works.

The Problem is not heavy but the Way to find the Source of the Problem is very heavy wy DNN displays here no detailed Informations whats wrong!!!