[Capt’n Flint]

I had the same problem and found the solution by coincidence:

most of the time I work with my old Wacom tablet (6 years+) and I alway got the flash. I used my normal mouse for another program and switched just for a second to AE, pushed the button (old habits die hard) and had to discover that the show-snapshot button works fine. that’s it. new input device good, old input device bad.

hope I could help you 🙂