Brian, could you replace your rules in your web.config with the following rule:
<rule name="ReverseProxyInboundRule1" stopProcessing="true">
<match url="^(file|link/|xas/|ws/|ws-doc/|sso/)(.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="Rewrite" url="http://localhost:8080/{R:1}{R:2}" />
</rule>
Issue resolved, by setting the default IIS ARR timeout to 240 seconds instead of the default 30 seconds.
Mendix Support (Matthijs Bloem) provided the following solution:
Start Internet Information Services Manager (IIS Manager)
In the IIS Manager select the "Server" in the Connections pane.
Open the Application Request Routing feature.
In the actions pane on the right hand side click on “Server Proxy Settings”
Adjust the "Time-out(seconds)" to your specific situation
Regarding your issue with tcp-ports in the header, have a look at the setting "Include TCP Port from Client IP" within the Custom Header section of the same form.