We have 2 sites with Exchange 2010, but I just introduce Exchange 2016 into the environment to run in co-existence. They have single name space "mail.domain.com/", but no I haven't made any DNS changes yet to point client services to 2016, only the certs is imported to 2016 and name spaces changed to match 2010. In site 1 when accessing the 2016 admin console, it works fine, I can click the icon for the 2016 admin console, and it opens the browser, I put in my credentials and I'm in the 2016 admin console. Also the 2016 powershell console works fine.
But in site 2 the powershell console works fine but the admin console doesn't work. It redirects me to the Exchange 2010 ECP. I haven't moved any mailboxes or anything. If i click the admin console icon, it opens to the admin console page, but once i enter my admin creds it redirects me. I have also tried using the servername (https://servername.domain.com/ecp?ExchClientVer=15), with the same results. I have cleared cookie cache, I have rebooted, and nothing seems to work.
So my mailbox is hosted on 2010 because no mailboxes have been moved yet, but in site1 everything is working correctly, but site 2 is having the issue. If i log into the console in site1 i see all the Exchange servers in site 2 like normal but strange that I can't access the console on a site 2 server.
Any advice on this by chance, before I have to engage Microsoft. See the screenshots. 1st 2016 admin console, 2nd 2010 ECP console
Try localhost/ECp also check if you have set the redirection on IIS