IIS ARR Exchange 2010 RPC over HTTPs timeout

Hello all,

I am trying to configure ARR on IIS7 as reverse proxy for our Exchange 2010 environment. Everything (owa,ecp,autodiscover) is working so far except RPC oder HTTPs.

The Failed Request Log shows the following:
ModuleName="ApplicationRequestRouting", Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="502", HttpReason="Bad Gateway", HttpSubStatus="3", ErrorCode="2147954402", ConfigExceptionInfo=""
ErrorDescription="...Timeout..."

My configuration:

1 IIS 7 on Server 2008 R2
ARR Proxy enabled with Response buffer 1536000KB and Respone buffer threshold 0KB
1 Serverfarm with the 2 Ex2010 CAS servers

My Rules look like this:

        <rewrite>
            <globalRules>
                <clear />
                <rule name="HTTP to HTTPS" enabled="true" patternSyntax="Wildcard" stopProcessing="true">
                    <match url="http://*" />
                    <conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
                    <action type="Redirect" url="https://{R:1}" />
                </rule>
                <rule name="ARR_HEX2010_loadbalance_SSL" enabled="true" patternSyntax="Wildcard" stopProcessing="false">
                    <match url="*" />
                    <conditions>
                        <add input="{HTTPS}" pattern="on" />
                    </conditions>
                    <action type="Rewrite" url="https://HEX2010/{R:0}" />
                    <serverVariables>
                        <set name="HTTP_ACCEPT_ENCODING" value="" />
                    </serverVariables>
                </rule>
            </globalRules>
            <allowedServerVariables>
                <add name="HTTP_ACCEPT_ENCODING" />
            </allowedServerVariables>
        </rewrite>

I was not able to activate winhttp tracing. Somehow the server is not creating any logfiles.
I am stuck on this error for a day now, so every help would be appreciated.

Best regards
Wilm