BEFORE you mess with your VNC, it is extremely important to have a backup connection. So either you have the ability to connect your pi to a monitor and a keyboard locally, or you really, really should setup SSH before you mess with your VNC server.

Use SSH with a Certificate, described here: https://raspberrypi-guide.github.io/networking/connecting-via-ssh (“passwordless”) This guide doesn’t show how to set up SSH, but how to install a key in a more detailed way: https://pimylifeup.com/raspberry-pi-ssh-keys/

The good thing: Once you got this working, you’re basically done. Just ditch VNC and go straight to SSH from now on. It’s more secure and has better performance usually.

Yet, if you like your VNC and want to continue using it, you first connect via SSH do not do this while using a VNC connection! Now, first, you do all this: https://www.tomshardware.com/how-to/install-vnc-raspberry-pi-os then you do a

sudo update-alternatives --list vncserver
sudo update-alternatives --list vncserver-x11

you should see tightvnc listed there. Don’t freak out if one of the two returns an error that the application was not found. That’s okay. Not all versions of Raspbian used the same application name in the past, so I listed them both. As long as one of them works, you’re fine.

Then, you do a

sudo update-alternatives --config vncserver
sudo update-alternatives --config vncserver-x11

and change it to tightvnc. now you can stop your running VNC:

sudo vncserver-x11 -service -stop && sudo vncserver -service -stop
sudo vncserver-x11 -service -start && sudo vncserver -service -start

Once you did that, connect to tightvnc as described in the article. If this works, do sudo apt uninstall realvnc

You should now be able to connect via VNC without weird account bullshit.