I'm hitting the same error too.
I'm using UltraVNC Viewer 184.108.40.206 on Windows 7. On this system I have VMWARE pro 12.1.1 and a virtual box running
I have enabled VNC connections on my Ubuntu box using the default port 5900 as is.
I can successfully ping from the host to the virtual and vice versa.
I can connect using putty (SSH) from the host to the virtual box. This confirms at least my IP address is correct.
I can't TELNET to this box. Just get connection failed.
I've tried with the firewall disabled on the host but got the same error.
VNC status is just "Failed to connect to server !"