I have run into a strange problem. I have a remote host running Windows 7. A VirtualBox VM instance is running Ubuntu.
The host runs a TightVNC server. The Ubuntu desktop is also VNC-enabled.
On a local Windows 7 system (a laptop), I use the TightVNC client to connect to either of the two systems.
If I connect to the Ubuntu VM directly, the ESC and Backspace keys work fine.
If I connect to the Windows 7 host, I cannot use ESC or Backspace in the Ubuntu guest. Nothing happens when these keys are typed; the key stroke events go to the proverbial bit bucket.
The keys work fine locally: the issue shows up only when accessing the Windows system remotely via VNC.
The issue is specific to the keys. I can obtain the ESC and backspace characters in the virtual machine by typing Ctrl-[ and Ctrl-H, respectively.
The issue is not specific to the keyboard software layout or hardware. Switching among languages using the Windows language bar makes no difference, and it makes no difference whether I use the laptop's built-in keyboard, or plug in a USB keyboard.
It is not a guest OS problem. The issue occurs not only in the Ubuntu machine, but in a VirtualBox instance running OS X, as well as a Solaris.
The VirtualBox GUI itself accepts the keys and processes them normally. For instance if I invoke Machine/Settings, I can dismiss that dialog with ESC.