Are you running virtualbox as administrator? You can also try moving your ova file to your desktop. That would eliminate a rights issue, as the ova is on a different disk. The ova may also be corrupted.
I had a similar problem recently, and fixed it by uninstalling Oracles Virtual Box then redownloading the lastest Oracle VB and the latest extension pack. Then restarting the ova file.
It seemed like the Virtual box software either did not reload the same ova title, or the Extension pack was the wrong version. Now my Virtual Box and extension pack is version 5.0.32. Hope this helps - - it was a real pain for me thou. Ken
It (VBox) finally wanted to upgrade so I let it. I am able to run the VM but I am having USB_BULK_SEND errors. Maybe I need to add whatever user to the dialout group?