If when trying to start a vm, for example using vagrant, you get the error message that the host network adapter cannot be created (the error is cryptic but will contain hostonly, create, vboxmanage -- unfortunately I forgot to screenshot it), make sure to first start the virtualbox GUI application. It does some extra checks, and sometimes highlights the permission problem:
In my case (OpenSuse 15.1 Leap), my user was not added to the vboxusers group. Via Yast > Users and Security .... and editing my user account it was a simple checkbox away. Remember to relog.