Ahh, I see the light now. The problem is because we should have that in the system.webserver section in the mojoportal Web.config. I did not know there was a setting that could be placed there but it makes total sense. As you have probably noticed there are a number of things in the system.webserver section that seem to be duplicates for things in the system.web section, like httpmodules and handlers, and apparently the custom error handling should also go there.
Basically its like this, IIS 6 will use the settings from system.web, and IIS 7 if configured with a "Classic" app pool will also use the settigns from system.web, but IIS 7 using the new default "Integrated Pipelione Mode" uses the settings from the system.webserver section and not the corresponding ones from system.web.
Even though I develop every day using IIS7 in Integrated pipeline mode I had not noticed any problem about the error redirect because I don't expect it to redirect when I'm working locally at localhost. But my production environment is IIS6 so my production sites were redirecting just fine and I never noticed that there should be this error section in system.webserver section.
I will add it now so it will be there for the next release.