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Check your /var/log/auth.log file. It’s possible that someone scanned and saw that you had a VNC server running (based on the port), and tried to brute force their way in. I’m not aware of any built in security on VNC, but it sounds like that’s why you’re getting the error.
To avoid this in the future, you can try setting up fail2ban. If you haven’t used it before, it’s a neat little piece of software that bans users based on their IP address after a certain number of failed attempts. If VNC is limiting the number of failed login attempts, this should help mitigate that.
You should also be connecting to VNC through an SSH tunnel, since it’s not a secure protocol. You can also try running it on a higher port that isn’t covered by generic nmap scans, although this is more supplemental and should not be relied upon for security.