This is a known bug and it will be corrected in the next version. There is a PHP warning that is causing the problem with the AJAX request. If you turn off the display of PHP errors it should work for you.
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ACF wouldn’t be so widely used in WordPress if it didn’t have some pretty amazing capabilities. In this article, we look at a few of the features we’ll discuss during “7 things you didn’t know you could do with ACF” at #WPEDecode later this month. https://t.co/5lnsTxp81jpic.twitter.com/Yf0ThPG1QG