So you added the first Exchange 2016 Preview server to your lab and now you want to access the Exchange admin center to configure your server. When you try to access https://<Exchange2016MailboxServer>/ecp and you enter your credentials you may see a ‘500 Unexpected Error’ or end up with the 2010 or 2013 version of the ECP. This is because Exchange 2016 by default tries to present the version of ECP that corresponds with the version of Exchange where your mailbox is hosted on.
To access the Exchange 2016 admin center while your mailbox is on an older version, append the string ?ExchClientVer=15.1 to your url. For instance https://<Exchange2016MailboxServer>/ecp?ExchClientVer=15.1.
Sounds familiar? That’s because the same procedure applied to Exchange 2013. Please note that the major version number of Exchange 2016 is 15.1, not 16 as you may have guessed.