Cannot move VirtualBox window

I find the same thing when imwheel is running, the utility that allows change to the mouse wheel. Holding down the Right-Control dows allow VirtualBox windows to be moved around.

For some reason, the auto capture of Virtual Box seems to conflict with the way how dragging a window from the title bar works

One easy workaround for this is to press the "Host key" (right control by default) to temporarily turn off auto capture mode, so you move the window around normally. (The status of keyboard capture, and the Host key, can be seen in the lower right of VirtualBox)