We are having the exact same problem.
We desactived every plugins but WPMLs and ACF Pro and we are getting that error also.
If we desactivate WPMLs and only let ACF Pro, the groups & fields saving is going perfectly. (except there’s no translation)
The website viability is compromised actually because of that bug…
We would like to have a solution or a workaround to get back on track asap.
-E
