WCF Url Rewriting using global.asax

I think maybe you should also ask in a wcf specific forum.

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Maybe it's the web config.

Are you switching sites and if so so you have:

<serviceHostingEnvironment aspNetCompatibilityEnabled="true"
  multipleSiteBindingsEnabled="true" />

If that's not the issue then maybe you can get more info with some failed request tracing:

http://www.iis.net/learn/extensions/url-rewrite-module/using-failed-request-tracing-to-trace-rewrite-rules

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