The exact same thing happened to me, and it was because I was trying to save my screenshot as a JPEG (as is default) when I experimented with an odd screen resolution (my old monitor had a 5:4 aspect ratio or something, not sure if AC supports/supported 1280x1024 natively).
Anyway, typing /screenshottype 2 will change the format in which your screenshots are captured from JPEG to PNG, which fixed the problem for me. You should then be able to just /screenshot or F12 as normal. Best of luck, hope it works out!
Anyway, typing /screenshottype 2 will change the format in which your screenshots are captured from JPEG to PNG, which fixed the problem for me. You should then be able to just /screenshot or F12 as normal. Best of luck, hope it works out!