When trying to access the Exchange Admin Center on either an Exchange 2016 or 2013 server with your Admin user account, you could potentially see an error (500 Unexpected Error) or get redirected to an older Exchange (2010 or 2013) and therefore not get access to the Administrative interface, that you were expecting.
This is by design, because you mailbox is still located on the older Exchange server.
The solution to Access the Exchange Admin Center (Exchange 2016 or 2013) is using the URL like this, appending (?ExchClientVer=15.1):
https://FQDNofTheExchangeServer/ECP?ExchClientVer=15.1
