Helicon ISAPI Rewrite Proxy 500 Internal Server Error

I have a website running at www.domain.com. The client now wants the website to appear to be running under www.otherdomain.com/whatson/brand/

Since the website is umbraco it won't run under a subfolder.

I wanted to use ISAPI rewrite to proxy requests to www.domain.com using the following rule in a .htaccess at www.otherdomain.com/whatson/brand/

RewriteRule ^(.*)$ http://www.domain.com/$1 [P,L]

However, when I apply this I get an ugly 500 Internal Server Error. There's nothing in the event log. So I turned on ISAPI logging and can see the following

111.111.111.111 111.111.111.111 Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (2) init rewrite engine with requested uri /whatson/brand/home.aspx

Then it testing all the other rewrite rules on the server.

Then this

111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (1) Htaccess process request w:\websites\otherdomain.com\docs2\whatson\brand\.htaccess
111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (3) applying pattern '^(.*)$' to uri 'home.aspx'
111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (2) forcing proxy-throughput with http://www.domain.com/home.aspx
111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (1) go-ahead with proxy request http://www.domain.com/home.aspx [OK]
111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (2) rewrite 'home.aspx' -> '/whatson/brand/home.aspxx.rwhlp?p=0'
111.111.111.111 111.111.111.111  Tue, 12-Jan-2010  13:05:24 GMT [www.otherdomain.com/sid#2045305275][rid#26337200/initial] (2) internal redirect with /whatson/brand/home.aspxx.rwhlp?p=0 [INTERNAL REDIRECT]

So it appears to work according to the logs, but I'm not seeing the page come through..

It's worth noting that www.domain.com and www.otherdomain.com are on the same box. LogLevel is 3 and RewriteLogLevel is 3 (I've tried with 9 and debug but there is too much traffic going through the other sites on the box)

Any ideas?