This error is not happening in mojoPortal code, that stack trace does not show any mojoPortal code.
The only times I have seen this error (I have heard of this error 2x before) is when you have an ISAPI Re-Write module installed on IIS 7 when it was designed for IIS 6. Those old dlls for IIS 6 write to the headers at a point in the request lifecycle that apparently IIS 7 does not like. So I suspect there is some re-write rule about login or register making it affect those urls.
If you have such a beast installed I would uninstall it and replace with the newer free Re-write module for IIS 7/7.5 prvided by the IIS Team via the Web Platform Installer.
Other than that make sure you are up to date on all windows updates and .NET 3.5 SP1.
Hope it helps,