Though this could seems an outdated post, I forgot its core so I will write it down again.
Sometime happen strange things. In the past days I faced a really weird issue with a migration from Exchange 2010 to Exchange 2016 for a customer.
I was at the point in which Exchange 2016 should be inserted as frontend server for all communications: all the load balancer Kemp (LAN and DMZ) was switched and all the checks was ok.
SMTP, ActiveSync, Outlook Anywhere, Autodiscover ECP and OWA was ok and the authentication between servers works fine. SMTP mail flow inbound and outbound moved to brand new servers. I also tested some mailbox migration and access to OWA and ECP.
That’s was fine, mailbox migration time! Customer’s IT responsible start to migrate some test mailbox before start a massive migration and after a couple of day they had problem to access to Outlook Web Access. It’s my turn to check which is the problem.