This is not a real question but I rather want to put this here in order to document it as there is a lot of contradicting information in the web.
When you run Virtualbox on a headless linux server, then there are afaik at least two native options (i.e. not RDP/VNC/X) to access Virtualbox remotely. One is phpvirtualbox and the other one is remotebox. Either of these solutions needs the virtual box web services.
Running the Virtualbox Web services as a root user is not a good option for security reasons. Many of the solutions which are suggested in the web are either outdated as they talk about changing the /etc/config/defaults files - but this is not relevant for systemd any more, just initd.
Other solutions only cover parts of the steps and fail for example with access rights to PAM or to the PID file.