A new version of RealVNC Server was made available in the repositories yesterday. If you have recently updated your packages and installed this new version, then you will need to restart any running VNC server instances.
It's probably easiest to simply reboot if you can, but if not you can restart the servers.
If you're using the service mode server (i.e. the one that lets you connect to the console session), do:
sudo systemctl restart vncserver-x11-serviced
For virtual mode servers (started via the "vncserver" command), you will need to shutdown the virtual session and restart it. Do:
vncserver -kill DISPLAY
Where DISPLAY is the display number (probably :1 if you have only one virtual mode server running).
and restart the server as normal with the "vncserver" command.