The VNC Viewer from RealVNC uses the "ColorLevel" pal8, which is not supported by most VNC servers.
In order to be able to connect successfully, please proceed as follows:
- Open the RealVNC options menu
- Click "Advanced options" in the bottom left corner
- Click on the "Expert" tab
- Change the "ColorLevel" value to full, rgb332 or rgb565
You should be able to successfully connect now ;-)