Question: Pre-6.5, I was able to install MBAM on any Server. After upgrading to 6.5 (or above) I am no longer able to install MBAM on my Servers.
Answer: MBAM installs was never supported on Servers, even pre-6.5, Kaseya just allowed you to install it on Servers without any restrictions.
On 6.5 (and above) we added the following restrictions when trying to install MBAM on a Server
There is no way to install MBAM on a Server after you upgrade to 6.5 (or above), a manual install is not supported by Kaseya. If you want to install an AV solution on a Server you will need to install Kaspersky (KAV) or AVG (KES).