Here's the config information:
[root@e-smith ~]# config show vboxweb-service
vboxweb-service=service
TCPPort=18083
access=localhost
status=enabled
[root@e-smith ~]# config show phpvirtualbox
phpvirtualbox=service
Group=
Name=phpvirtualbox
URL=VB
User=
status=enabled
webauth=enabled
sorry for the delay of my answer, all above looks fine.
I don't know/understand what is occuring, I suppose that your problem is not fixed and after a reboot or a reconfiguration the vboxweb-service must be restarted. Right ?
In fact that service is not a part of phpvirtualbox, but from virtualbox, omething has changed between the version 4.3 to 5. Before the service was launched at S35 without problem.
maybe a (bad) trick could be done in that case of non opened port, put in /etc/rc.d/rc.local : /etc/init.d/vboxweb-service restart
keep me in touch