Not working on a Win7 host !!
Since we have installed VMware Workstation pro 15 on our Win7 host, the Virtualbox (6.0.4) guests have their internet connection disconnected/reconnected every few seconds.
(Not running Virtualbox and VMware at the same time.)
After uninstalling VMware Workstation, the Virtualbox guests work fine again.
On a Win10 host both work fine so far.