The maximum number of server(s) can be installed with this serial number has been reached

You may experience the following error:

[ERROR] [LICENSE] License key operation failure: DT.7: The maximum number of server(s) can be installed with this serial number has been reached. If you want to migrate a license used on another server, please consult the product manual regarding how to transfer a license. count=1

which means you may have used the same serial no for another server. A serial no can only be used on one server at a time. Most likely you are transferring your license from an old server to a new server. How the transfer to be done?

Please check wiki here for steps to be followed.