You can also try this,
worked for me after fresh install of OMV 4
Keep in mind that it will remove VNC, but RDP and console is working perfectly after installing this.
Check Vbox version upfront, newest from OMV Extra repo is Vitualbox 5.2.24:
Upgrade Ext Pack:
Remove VNC, use RDP instead:
Maybe you can skip the update, but I didn`t try