After having worked without problems for a while, the GUI now no longer shows up when starting VirtualBox.exe. However, the process runs & can be killed.
At the same time, I noticed VBoxSVC.exe runs and cannot be killed (tried from the Windows Task Manager and sysinternals' Process Explorer running as administrator). I'd guess this problem is related to the first.
I found a pair of threads describing the same issue, about being unable to kill VBoxSVC.exe and the VirtualBox.exe GUI not showing, but they don't contain much useful info (e.g. I have no wireless adapter, as the last thread suggests might be the culprit).
How can I debug this problem and/or how can I help you reproduce the problem?
Environment:
VirtualBox 5.0.10
Host OS: Windows 7 Professional x64 (Microsoft Windows [Version 6.1.7601])
Guest OS: I had created 2 VMs, one of Fedora 23 and one of the Solaris 11.3 pre-built .ova