Ideas
I have also had this happen a couple of times, though it doesn't happen all the time.
The first time it happened I was playing in a mod (and cc) free save and the first screenshot was fine, but the second time I took a screenshot (right afterwards) my game froze and I had to alt tab out and open task manager to close the game. I restarted my computer and then using c worked for a while, using it didn't freeze my game for a couple of weeks.
The second time I had it happen I did have mods in and again I had to alt tab out and use task manager to close the game (and the message in task manager both times was that The Sims 4 was not responding). Note that both times I was just playing regularly in live mode, I was not in POV or tab mode.
I play on Windows 10. Both times was on game version PC 1.118.257.1020 DX 11 and both times my sims were in the process of aging up on a birthday cake (that's often when I tend to get in game screenshots).
Best regards,
Jennifer