After latest update to 3.9.14, started to get nonce errors when uploading images:
No other changes to site other than updating ACF Front End.
Does not happen on back end.
Tried logging out and logging in.
Tried adding new users.
Clearing browser and hosting cache.
Tried on different browsers on different computers.
Then updated to WordPress 6.1.1 to refresh core files, made no difference.