"Failed to execute URL" using Elmah in ASP.NET MVC 1.0

I added Elmah to my ASP.NET MVC (1.0) web application.

The / elmah.axd requests work fine, but they do not display correctly. A little digging shows that the request /elmah.axd/stylesheet is causing an HTTP 500 error:

Failed to Execute URL.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: Failed to Execute URL.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 

[HttpException (0x80004005): Failed to Execute URL.]
   System.Web.Hosting.ISAPIWorkerRequestInProcForIIS6.BeginExecuteUrl(String url, String method, String childHeaders, Boolean sendHeaders, Boolean addUserIndo, IntPtr token, String name, String authType, Byte[] entity, AsyncCallback cb, Object state) +2003857
   System.Web.HttpResponse.BeginExecuteUrlForEntireResponse(String pathOverride, NameValueCollection requestHeaders, AsyncCallback cb, Object state) +393
   System.Web.DefaultHttpHandler.BeginProcessRequest(HttpContext context, AsyncCallback callback, Object state) +220
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8682818
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

I checked the routing using the Phil Haack RouteDebug.dll file and there were no problems - the Elmah request matches routes.IgnoreRoute("{resource}.axd/{*pathInfo}");in Global.asax.csas it should be - and I can't seem to find more information about what causes the error.

I get the same “URL Failed” from any of the detailed links in Elmah, for example:

http://mysite/myMvcApp/elmah.axd/detail?id=FDA51223-4486-4759-9075-3C5DAE82094B

Any ideas?