sierra lock screen on remote desktop

I have a mac pro 5.1 that I recently upgraded to sierra. The machine is headless, because it is acting as a server.

Before the upgrade I could access the screen of the machine through the remote desktop app. If the screen was locked I could unlock it with a user's password.

Since the upgrade, if the screen gets locked when I connect with remote desktop I see a black screen and a lock flashes imediately after connecting.

I tried to configure a password-less login for the single user on the machine, but occasionally the screen gets locked anyway.

How can I prevent the screen locks ? Or alternatively be able to see the lock screen through remote desktop to remotely unlock it ?

Thanks