I am also experiencing this out of the blue phenomenon on one of my installs (creating new fields and not being able to save their data). At first I thought it was WPML causing this, however upon their investigation, they are suggesting that ACF is the culprit. Not sure what to do from here, tried the Beta version and still no luck. I am manually updating the DB to perform customer requests in the meantime, but of course this is not a viable solution.
I have removed plugins, deactivated / reactivated ACF, scoured for answers, implemented suggestions etc. etc. and same results.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!