too many authentication failures

VNC has a blacklisting system built in, so the blacklist will stop VNC login attempts after a certain number fail. These attempts may be legitimate, such as a user who accidentally forgot their password, or illegitimate, such as bots attempting to gain access via brute force. This is a security measure designed to protect your server. The blacklist will reset on its own after some time goes by without login attempts from the blacklisted IPs, but you can also force a reset by restarting VNC. This blog post from VNC's site helps to explain this feature. You can also prevent this issue in the future by limiting access to VNC via firewall.