If you are not able to connect vnc from panel and it throws ‘Failed to connect to server‘
Then firstly stop and start vps from panel if vnc was enabled after the vps was created.
If vnc was enabled while creating vps then make sure firewall service is not blocking 4081 and the vnc port(59XX) which that vps is using.
You can view vnc port from manage vps page while accessing vnc.
If firewall is not blocking then check if websockify service is running or not :
ps aux |grep websoc
You will need socat package installed on that server and then try to access vnc from panel and check again for websoc process on master server.
Note : Make sure ‘ use master proxy ‘ is off in Configuration->Master settings.
Also correct server name is present in Servers->List server page which resolves to an ip.
Then check if ‘ use server hostname ‘ for that slave server in slave settings and also in master settings.