Right-click Discard Saved State... to fix E_FAIL (0x80004005) on Version 5.2.12 r122591 (Qt5.6.2).
If you see this error:
Right click on the virtual machine you tried to start:
...and click Discard Saved State...
Then start the virtual machine.
Text of Error
VirtualBox - Error
Failed to open a session for the virtual machine machine-name.
The VM session was closed before any attempt to power it on.
Answer to https://www.virtualbox.org/ticket/6930 by Democritus
VirtualBox logo from [link]