glsf91 could you please explain in detail what settings you are using in the KeyboardInputPlugin for that key that doesn't work (what action, condition, for a specific plugin or general fullscreen video, ...).
Maybe I can reproduce it and fix it. But I can't promise anything.
For example I have de key code 10079 (Control O) added to button name Info (for context menu). Condition Fullscreen and action context menu. Also a layer with no condition and action context menu.
When I then play a .mkv file start/stop, play again then this key was not working anymore. If I press F9 (MP context key defined) it show the context menu ! Sometimes it works, mostly not.
When I disable this plugin en define in MP config, General, Keys and sounds this key Ctrl O with ACTION_CONTEXT_MENU, it works without problems.
With MP 1.0.2 on XP and plugin 2.2.0.0 no problem at all.
I added the file I used.