Office Online Server : We couldn't find the file you wanted. It's possible the file was renamed, moved or deleted

Question

Environment: On-premise SharePoint 2016 On-premise Office Online Server 2016

1. Installed Office Online Server Mar 2017 Update on to a 2012 R2 Server

   Verified OOS is working using

   https :// myOOSserver.com /hosting/discovery
   https :// myOOSserver.com/x/_vti_bin/ExcelServiceInternal.asmx
   using Certificate: OOSCertificate
   

2. Configured Office Online Server for SharePoint 2016

Followed https :// technet.microsoft.com/en-us/library/ff431687(v=office.16).aspx

   Set-SPWOPIZone -zone "internal-https"

    using Certificate: SP2016Certificate

3. Configured server-to-server authentication between Office Online Server and SharePoint Server 2016

   followed:

   https :// technet.microsoft.com/en-us/library/mt346470(v=office.16).aspx

   exported OOSCertificate and used OOSCertificate on SharePoint server to set up server 2 server auth.

4. In a new Site collection > Documents Library, Added a new Excel Workbook > it redirects to https :// mySP2016Server.com/_layouts/15/WopiFrame.aspx?sourcedoc=XXXXXXXXXXXX

momentarily shows... Excel Online
but then shows the model pop up

Unable to Open the File We couldn't find the file you wanted. It's possible the file was renamed, moved or deleted.

inspected using Dev Tools:

https :// mySP2016Server.com/_layouts/15/WopiFrame.aspx?sourcedoc={GUID-GUID-GUID-GUID-GUID}&file=Book.xlsx&action=default

Status: 200

https :// myOOSserver.com/x/_layouts/xlviewerinternal.aspx?ui=en%2DUS&rs=en%2DUS&WOPISrc=https%3A%2F%2FmySP2016Server%2Ecom%2F%5Fvti%5Fbin%2Fwopi%2Eashx%2Ffiles%2FGUID&wdEnableRoaming=1&wdo=1

Status: 200

I suspect there is an issue between server to server communication. Can someone please help?

Thank you.

PS: 1. prior to installing March 2017 update, we had Nov 2016 install of OOS 2. it was set to "internal-http: 3. everything was working fine