Please see
http://www.virtualmin.com/node/12251#comment-124877
I can not get errors to pass to the browser and I think it has something to do with fcgid
The thread you linked to is very long. You might want to reiterate your exact problem here and tell us step-by-step what you did and what errors occurred.
as Gilda Radner would say: Never Mind
I took a break for a few hours and now everything works, even the local manual error html.