Login on Putty SSH using root Access and type the following commands.
Step 1: type the commands pgrep vnc and there will be show some number bellow after you type the commands.
Step 2: the number show is the number ID of the running VNC on the system. so you need to kill it to close all running vnc by typing kill ID.
Step 3: type reboot to reboot your system.
After that you can now allowed to use the VNC without the Authentication. if this solution not work
Open a Ticket Support or message on Live Chat System.
Thank you ^_^