URL Rewrite Module - Error 500.52

Hi,

I have a problem with URL Rewrite. Here are my rules:

<rewrite>
<rules>
<rule name="ReverseProxyInboundRule3" enabled="true" stopProcessing="true">
<match url="(.*)" />
<action type="Rewrite" url="http://localhost:81/{R:1}" />
</rule>
</rules>
<outboundRules>
<rule name="ReverseProxyOutboundRule3" preCondition="ResponseIsHtml1" enabled="true">
<match filterByTags="A, Form, Img" pattern="^http://localhost:81/(.*)" />
<action type="Rewrite" value="http{R:1}://10.41.13.11/api{R:2}" />
</rule>
<rule name="Rewrite Location Header" preCondition="IsRedirect" enabled="true">
<match serverVariable="RESPONSE_Location" pattern="^http:/(.*)" />
<action type="Rewrite" value="http://testxxx.zzzz.com/api/{R:1}" />
</rule>
<rule name="relativeURL">
<match filterByTags="A, Form, Img" pattern="^/(.*)" />
<action type="Rewrite" value="/api/{R:1}" />
</rule>
<preConditions>
<preCondition name="ResponseIsHtml1">
<add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
</preCondition>
<preCondition name="IsRedirect">
<add input="{RESPONSE_STATUS}" pattern="3\d\d" />
</preCondition>
</preConditions>
</outboundRules>
</rewrite>
<tracing>
<traceFailedRequests>
<add path="*">
<traceAreas>
<add provider="WWW Server" areas="Authentication,Security,Filter,StaticFile,CGI,Compression,Cache,RequestNotifications,Module,FastCGI,Rewrite,RequestRouting" verbosity="Verbose" />
</traceAreas>
<failureDefinitions timeTaken="00:00:00" statusCodes="400-999" />
</add>
</traceFailedRequests>
</tracing>

Error:

 The service on port 81 is NOT using compression.

This IIS is sitting behind a WAF so the request will be proxied twice, can that be a problem?
I have used RawCap to analyze the response that gets back to IIS from the service on port 81, and there is nothing wrong with it that I can see.
 

Thanks,
Andreas