Issues with Classic ASP on IIS7

(generic tips since user updated question as resolved)

In the future, hit the page in question FROM the server it is occuring on (if possible). This forces IIS7 to give a much more detailed view of what is going on. So long as you have local detailed errors on, you will get a screen with extensive details instead of just HTTP 500. This can show you the ISAPI Provider that honored the request. 9 times out of 10 that will show you what's happening and the failure will reveal itself.

If not, turn on "failed request logging". IIS will then dump every peice of data it has when a 500 class error is thrown. Its taxing to go through, but if you really want to know, thats the best way.

I found this thread while searching for the exact same error description. And I found what was causing it in my case.

I was converting an application from .net 3.5 to 4.0 and after I changed the app-pool from 2.0 to 4.0 this started. It only happened in classic mode not integrated.

In my case I had wildcard http handlers in web.config:

<add name="Wildcard .net 64 bit" path="*" verb="*" modules="IsapiModule" scriptProcessor="C:\Windows\Microsoft.NET\Framework64\v2.0.50727\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="None" preCondition="bitness64"/>    
<add name="Wildcard .net 32 bit" path="*" verb="*" modules="IsapiModule" scriptProcessor="C:\Windows\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="None" preCondition="bitness32"/>

But I had forgotten to change the path to the .net 4.0 directory

so after changing \v2.0.50727\aspnet_isapi.dll to \v4.0.30319\aspnet_isapi.dll the website started behaving normally.

Hope this helps someone.