guest and VB Manager GUI hang after guest startx

[RossB]

SYMPTOMS
Both my guest VM and the VB GUI on the host appear to have hung. This happened when I executed startx to launch a graphical environment in the guest; the display expanded to fill the available space, but it was not responsive to mouse clicks or insert ctl-alt-backspace from the menu. The display is simply a blank window, which is what it would be if things were working. When I resize the outer window, the inner window does not resize with it, as it does when things are working. The decorations and menus around the guest window continue to be responsive, in that they are displayed and clickable.

The controlling VB GUI on the host (that is, the GUI launched by typing VirtualBox, that controls all VMS) appears hung. The window is there, but it is simply a grey square.

Attempts to ssh into the guest yield "No route to host".

VirtualBox is using a small amount of CPU. VBoxManage is able to list information.

MAIN QUESTION
What's the safest way to recover from this?

SECONDARY QUESTION
Why did this happen and how can I avoid it in the future?
I plan to reboot the host, but at the moment I'm concerned shutting down could corrupt the VM.

CHRONOLOGY
The VM is/was running mythtv, and was working fine.
Updates to the host included the kernel. I did not reboot the host.
Updates to the VM included the kernel; I rebooted the VM.
The VM ran successfully headless, i.e., myth recorded shows and served them to other machines.
Today I logged into the VM on the host, using the window VB opened when the guest started.
I expanded the window and executed startx. This is where things hung.

BACKGROUND
Host is running Debian wheezy (oldstable) with virtualbox-qt 4.3.32-dfsg-1+deb8u2~bpo70 amd64. I am using nvidia proprietary drivers, locally built to fit the newest drivers into the oldish software environment.
kernel is 3.16.0-0.bpo.4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u3~bpo70+1 (2016-01-19) x86_64 GNU/Linux

Guest is running Debian jessie (stable). Since I can't access it specifics are hard, but it's pretty current. I believe it's using the Debian packaged guest additions.

Multicore Intel hardware underneath.

I've looked around for something in the logs (system logs as well as the one attached), but see nothing.

[mpack]

You are running the Debian fork of VirtualBox. I'm afraid we only support actual VirtualBox on these forums.

Your options are to either install VirtualBox and verify that the problem still exists, or ask for support on the Debian forums.

[RossB]

I rebooted the host system, and everything seems OK now, though the guest reported it had not been shutdown cleanly and did some disk cleanup.

Before the reboot I tried various VBoxManage controlvm commands (pause and savestate), but the commands just hung, i.e., I did not get a prompt back until I did ctl-C. I also tried similar commands from the VM window.

I tried to kill the VB Manager window, but it did not respond (maybe because it was running as root?).

The VM was in the "abort" state after the reboot, but was startable.

I'm not exactly sure what the lesson is here, but I should probably go back to me old practice of not upgrading the host VM while VB is running.

I'm sorry to hear you say you don't support distro-specific packages here. I expect there's a lot more knowledge about VB here than in the distributions.

[mpack]

They are third party forks, possibly modifed in ways we don't know about. They are not "distro specific packages" - you can download those from here. It is really quite simple: if you want VirtualBox support, you have to be a VirtualBox user.