IIS Rewriting dosnt allow/breaks post requests - post

Question

I have an IIS instance in front of my other non-iis web server hosting the site where the IIS is doing some AAR magic which is working good but for POST requests this dont seem to work for some reasons which mean any ajax calls by the application on the non IIS webserver end up broken. Any ideas on how to solve this? Below is the current rule setup.
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<httpRedirect enabled="false" />
<rewrite>
<rules>
<clear />
<rule name="UrlWithPimRewrite" stopProcessing="true">
<match url="pim.*" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{CACHE_URL}" pattern="^(https?)://" />
</conditions>
<serverVariables>
<set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" />
<set name="HTTP_ACCEPT_ENCODING" value="" />
</serverVariables>
<action type="Rewrite" url="{C:1}://localhost:1512/{R:0}" appendQueryString="true" logRewrittenUrl="true" />
</rule>
<rule name="UrlWithoutPIM" enabled="true" stopProcessing="true">
<match url="^(.*)$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{URL}" pattern="/pim" negate="true" />
<add input="{CACHE_URL}" pattern="^(https?)://" />
</conditions>
<serverVariables>
<set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" />
<set name="HTTP_ACCEPT_ENCODING" value="" />
</serverVariables>
<action type="Rewrite" url="{C:1}://localhost:1512/pim/webaccess" appendQueryString="false" logRewrittenUrl="true" />
</rule>
</rules>
<outboundRules>
<clear />
<rule name="RestoreAcceptEncoding" preCondition="NeedsRestoringAcceptEncoding" enabled="true">
<match serverVariable="HTTP_ACCEPT_ENCODING" pattern="^(.*)" />
<action type="Rewrite" value="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" />
</rule>
<preConditions>
<preCondition name="NeedsRestoringAcceptEncoding">
<add input="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" pattern=".+" />
</preCondition>
</preConditions>
</outboundRules>
</rewrite>
<urlCompression doStaticCompression="true" doDynamicCompression="true" />
<directoryBrowse enabled="false" />
<tracing>
<traceFailedRequests>
<add path="*">
<traceAreas>
<add provider="ASP" verbosity="Verbose" />
<add provider="ISAPI Extension" verbosity="Verbose" />
<add provider="WWW Server" areas="Authentication,Security,Filter,StaticFile,CGI,Compression,Cache,RequestNotifications,Module,FastCGI,WebSocket" verbosity="Verbose" />
</traceAreas>
<failureDefinitions statusCodes="404" />
</add>
</traceFailedRequests>
</tracing>
</system.webServer>
</configuration>