OK, question was answered in the FAQ (when all else fails, RTFM I guess)
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Why does taking screenshots with F3 not work?
The video plugin's internal screen capture may not work with FSAA enabled. If running in a window, you'll receive an error message. If running fullscreen, there's no error message because that would interrupt your game. One workaround is to turn off Anti-Aliasing, or use PrtScrn or some 3rd party capture utility. The other would be to use your graphics card's software to force anti-aliasing, instead of the plugin slider.
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