ExRCA testing issue

we're using ExRCA to test the external accessibility to the internal exchange servers ( 2 servers in DAG )

The firewall is configured with "simple" DNAT with no protcol inspection.

Th error we get while testing for Outlook connectivity with autodiscover.

If we test the autodiscver url directly

https://autodiscover.domain.com/autodiscover/autodiscover.xml

we're prompted for user and password and when we insert them we got 

<Autodiscover>
<Response>
<Error Time="10:54:33.7520471" Id="3179205853">
<ErrorCode>600</ErrorCode>
<Message>Invalid Request</Message>
<DebugData/>
</Error>
</Response>
</Autodiscover>

what can we check for ?