You may think this error was a result of upgrade but I do not think so. This error is not pointing to any mojoportal code, it is pointing to ASP.NET runtime code and there is no mojoportal code that touches the http header collection.
I believe you have some 3rd party isapi module probably an url rewriter installed that is not compatible with IIS 7 or else not compatible with IIS 7 in integrated pipeline mode.
You "may" be able to solve it by changing the app pool for mojoPortal to a "Classic" app pool rather than the default "Integrated" app pool.
Probably the best solution is to uninstall this isapi filter and use the free url rewrite module that is now available for IIS 7.
Search for the Web Platform Installer, install it and you will see that it can install the free url rewriter module.
Hope it helps,