I experience the same problem as described above. I tried the suggestion to switch on the "VirtualBox NDIS6 Bridged Networking Driver" as suggested. But this one is already on and a change does not fix the problem. Could you please support on this issue?
The error message that Virtural Box gives is:
Failed to open/create the internal network 'HostInterfaceNetworking-VirtualBox Host-Only Ethernet Adapter' (VERR_INTNET_FLT_IF_NOT_FOUND).
Failed to attach the network LUN (VERR_INTNET_FLT_IF_NOT_FOUND).
I have seen this problem in a class I had conducted before, and with everything we tried, nothing seems to resolve it. Strangely the problem does not exist on another student's machine with exactly the same specification! We were finally able to resolve it by installing and using VMware Player instead of VirtualBox.
I also noted you are using 5.0.20. Perhaps you can try to update it to the latest version 5.1 version and see if it helps to resolve the issue.