This is also happening to me too. Zorin 16.2 is on both the host and client for me and I am using Remmina as my client. I can successfully connect to the PC after setting up "screen sharing" setting and setting a password. After a while though, It will no longer work. Both Zorin machines have reserved IP addresses on a LAN network. The VNC connection seems to not work when the computer is locked and not showing the desktop. Yetimaan, could you make the PC never go to sleep to see if that works?
Remmina is installed on Zorin OS by default. Its repository is ip_zorinos_apps-focal-main. I'm sure it has all the permissions it needs. It seems he is having connection issues created by server side. You can try two things yetimaan, plug in a monitor over HDMI on your server (skip this if your graphics card has a VGA port). Set the screen off/lock time to never on your server. If you wouldn't mind, let me know if these work. I am having the same issue.