Re: 500 and 404 errors on IIS 7.5
Have you tried to use your WPMU alone at first ?
I recommand you to test anything in WPMU (creating blogs, users, sending mails, etc)
If all is ok, install buddypress and deactivate all the 7 BP plugins (buddypress ->options) exept one . View if it’s working, activate the second, and so on…