This generally suggests a network issue which is not related to the VNC® application itself. This could be due to network hardware being either misconfigured or faulty.
As far as the application reporting the error is concerned, the other endpoint sent a TCP reset packet. Sometimes, both endpoints report this error, in which case something in the middle has sent reset packets to both ends. It's not possible for the endpoints to get any more information than this. That unfortunately means the VNC Viewer cannot tell what has caused the error.
If you wish to troubleshoot this issue, investigating the quality of the connection between viewer and server is the best place to start. Try sending large ping packets and do a reasonably long ping test, looking for any packet loss. A payload of around 1350 bytes is ideal.
From a command prompt:
ping -t -l 1350
Note: it is a "dash lowercase-L" above, not a 1, they look the same in some fonts.
One common cause of this error is that the MTU (Maximum Transmission Unit) size of packets travelling over your network connection have been changed from the default of 1500 bytes. This could have been configured manually (usually within router settings), or could be due to running VNC over a VPN connection.
Using fixed IP addresses rather than DHCP assigned addresses on your router settings has been known to resolve this issue.