Some one configured my server in a way that intentionally or inadvertently blocks ELMAH. I wish I know what that was. My current server admin can't figure out what's blocking it either.
Clues have so far
Error message is "This type of page is not served...it has been explicitly forbidden"
Ordinary .ashx handlers work just fine. (HelloWorld.ashx works)
I'm running in Full Trust.
I'm on Win 2000, IIS 5
Machine config doesn't have any references to ELMAH (neither does my web.config)
The ELMAH_Error table exists, but has no entries.
My application with ELMAH works fine on my machine (i.e. shows the error log).
I updated the Handler section to change the name to Elmha.ashx, instead of elmah.axd.
This was happening on server I didn't have access to the windows application error log. After getting that, it was obvious that elmah and blowery were not playing well together (headers already sent errors).