If these php warnings are being generated during the ajax request then the text of these warning is what the ACF ajax request is getting back instead of what it should be getting. You need to clear up these errors.
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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