[SOLVED] ApplicationRequestRouting can't connect to URL but the server's browser can

ApplicationRequestRouting can't connect to URL but the server's browser can

I use ApplicationRequestRouting and UrlRewrite v7.1.1980 in IIS to create a reverse-proxy to our backend, but it returns 502.3 Bad Gateway:

<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
    <System>
        <Provider Name="WWW Server" Guid="{3A2A4E84-4C21-4981-AE10-3FDA0D9B0F83}"/>
        <EventID>0</EventID>
        <Version>1</Version>
        <Level>3</Level>
        <Opcode>16</Opcode>
        <Keywords>0x100</Keywords>
        <TimeCreated SystemTime="2022-10-24T10:16:29.431Z"/>
        <Correlation ActivityID="{80000011-0002-F900-B63F-84710C7967BB}"/>
        <Execution ProcessID="6684" ThreadID="4080"/>
        <Computer>Computer1</Computer>
    </System>
    <EventData>
        <Data Name="ContextId">{80000011-0002-F900-B63F-84710C7967BB}</Data>
        <Data Name="ModuleName">ApplicationRequestRouting</Data>
        <Data Name="Notification">128</Data>
        <Data Name="HttpStatus">502</Data>
        <Data Name="HttpReason">Bad Gateway</Data>
        <Data Name="HttpSubStatus">3</Data>
        <Data Name="ErrorCode">2147954429</Data>
        <Data Name="ConfigExceptionInfo"/>
    </EventData>
    <RenderingInfo Culture="en-US">
        <Opcode>MODULE_SET_RESPONSE_ERROR_STATUS</Opcode>
        <Keywords>
            <Keyword>RequestNotifications</Keyword>
        </Keywords>
        <freb:Description Data="Notification">EXECUTE_REQUEST_HANDLER</freb:Description>
    </RenderingInfo>
    <ExtendedTracingInfo xmlns="http://schemas.microsoft.com/win/2004/08/events/trace">
        <EventGuid>{002E91E3-E7AE-44AB-8E07-99230FFA6ADE}</EventGuid>
    </ExtendedTracingInfo>
</Event>

However, when I log into the server with Remote Desktop and open our URL in the server's browser, it opens fine - so there is connectivity between our server and backend.

How can the browser "see" the URL but IIS claims it has no connection?

Solution

We found the issue. Inside of our ARR Cache config we had added a proxy server which had stopped responding. After removing the proxy server, it works again.