Iis – ASP detailed error messages on IIS 7.5

I'm having real issues getting some (externally-written) ASP running on IIS 7.5.

ASP is set up and I've enabled error messages in the two places mentioned in articles such as http://weblogs.asp.net/nannettethacker/archive/2009/01/09/enabling-asp-classic-error-messages-in-iis7-and-windows-web-server-2008.aspx, however I'm not getting proper error messages.

As an example, this ASP page works as expected (on-screen shows "hello world"):

hello
<%
Response.Write "world"
%>

Interestingly, if I force a compile error, I get detailed errors:

hello
<%
Response.Write "world";
%>

Produces:

Microsoft VBScript compilation error '800a0401' 
Expected end of statement 
/test.asp, line 3 
Response.Write "world";
----------------------^

However, if I change the code so it produces a runtime error, it silently fails:

hello
<%
Dim x: x = Int("hello world")
Response.Write "world"
%>

This will output:

hello

i.e. no error, no "world", no nothing. Tracing the request/response in Fiddler shows that it just seems to stop at this point. I can't see anything on the server that's logging an error.

Been tearing my hair out on this for a while – I just can't see what's swallowing the error.