I am in the process of migrating from Exchange 2010 to Exchange 2016 and ran into an issue with client connectivity. Both servers are on the same network with external ports 25, 443 and 80 forwarded to Exchange 2010.
Once I move a mailbox to 2016 I can no longer access it using Exchange 2010 owa page and Outlook clients lose connectivity as well.
When I try to login to a mailbox that was created on Exchange 2010 using Exchange 2016 local OWA page I get "(HTTP 500 Internal Server Error) means that the website you are visiting had a server problem which prevented the webpage from displaying. "
When I try to login to a mailbox that was created on Exchange 2016 using Exchange 2010 OWA page I get "A server configuration change is temporarily preventing access to your account. Please close all Web browser windows and try again in a few minutes. If the problem continues, contact your helpdesk."
On Exchange 2016 I setup virtual directory URLs to point to mail.mydomain.com. On 2010 I removed External directory URLs. SSL certificate was exported from 2010 and imported into 2016. I checked authentication methods for both 2010 and 2016. They are set for NTLM and negotiate.