If you are planning to customize and extend mojoportal you should
get the source code from svn trunk, rebuild the solution, set your connection string and try stepping through the code if the problem persists (it may go away if you get the code from svn).
Other than that I can try and see if I can replicate the problem but it may be a few days before I get a chance.
The error indicates a line of code that is trying to get the siteSettings object from the http context and it turns out to be null when it should have been put in the http context in global.asax application_beginrequest.
Other clues/error messages may be available in ~/Data/currentlog.config file
Hope it helps,
Joe