I recently started playing FFXI in 4K, and Windower fails to take screenshots when running at this resolution.
Pressing print screen will result in 3 different possible outcomes:
1. The game crashes. This happens most often. You get this error: http://i.imgur.com/ecyugtl.png
2. Windower console states that it "failed to convert to png", doesn't take the screenshot.
3. It takes the screenshot, but ONLY if the screenshot result would be a small file size (loading screen with everything black).
I tried doing the command "screenshot jpg hide", and it resulted in a .bmp screenshot, but still crashed the game.
EDIT: Game also randomly crashes, so I guess I won't be doing this anymore.