I've just encountered the issue too.

As an alternative to aibrahim's method if you are doing it remotely via ssh, here's my steps to VNC to display :0.

1. Setup password using sudo vncpasswd -service
2. Add the following lines to /root/.vnc/config.d/vncserver-x11 as described in RealVNC: Support legacy viewers

   SecurityTypes=VncAuth
   UserPasswdVerifier=VncAuth
   

3. Start vncserver using command sudo vncserver-x11-serviced

This method allows me to login using Ubuntu's Remmina VNC client.

I've just encountered the issue too.

As an alternative to aibrahim's method if you are doing it remotely via ssh, here's my steps to VNC to display :0.

1. Setup password using sudo vncpasswd -service
2. Add the following lines to /root/.vnc/config.d/vncserver-x11 as described in RealVNC: Support legacy viewers

   SecurityTypes=VncAuth
   UserPasswdVerifier=VncAuth
   

3. Start vncserver using command sudo vncserver-x11-serviced

This method allows me to login using Ubuntu's Remmina VNC client.

I've just encountered the issue too.

As an alternative to aibrahim method if you are doing it remotely via ssh, here's my steps to vnc to display :0

1. Setup password using sudo vncpasswd -service
2. Add the following lines to /root/.vnc/config.d/vncserver-x11 as described in RealVNC: Support legacy viewers

    SecurityTypes=VncAuth
    UserPasswdVerifier=VncAuth

3. Start vncserver using command sudo vncserver-x11-serviced

This method allows me to login using Ubuntu's Remmina VNC client.

I've just encountered the issue too.

As an alternative to aibrahim's method if you are doing it remotely via ssh, here's my steps to VNC to display :0.

1. Setup password using sudo vncpasswd -service
2. Add the following lines to /root/.vnc/config.d/vncserver-x11 as described in RealVNC: Support legacy viewers

   SecurityTypes=VncAuth
   UserPasswdVerifier=VncAuth
   

3. Start vncserver using command sudo vncserver-x11-serviced

This method allows me to login using Ubuntu's Remmina VNC client.