URL Rewrite gives 500.50 error for 401 URLs

I'm trying to trap 401 errors that are returned when people try to access files in certain directories on the server (win 2008 iis 7.5).

With no web.config, the user sees a 401 as expected.

I am trying to trap and redirect this error but not having any luck. Getting a 500.50 error instead.

Here's my web.config, which works well in all other respects.

What am I doing wrong?

Thanks for any reply

<?xml version="1.0" encoding="UTF-8"?><configuration><system.webServer><rewrite><rules><rule name="Imported Rule 1" stopProcessing="true"><match url="^(.*)$" ignoreCase="true" /><conditions><add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="true" negate="true" /><add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="true" negate="true" /><add input="{URL}" pattern="^/favicon.ico$" ignoreCase="true" negate="true" /></conditions><action type="Rewrite" url="/index.cfm?RW404={R:1}" appendQueryString="true" /></rule></rules></rewrite><httpErrors errorMode="Detailed"><remove statusCode="401" /><error statusCode="401" prefixLanguageFilePath="c:\website" path="error.htm" /></httpErrors></system.webServer></configuration>