There have been others that have had this problem in the past. As far as I can remember it is always a compatibility problem with some other plugin. I don’t have an answer for you but I have some things to check if you haven’t already done so.
What version of ACF is active? I’m unfamiliar with the Gallery field in ACF4. If you’re using 4 check the field setting to make sure they are correct.
Has anything else besides WP been updated recently?
Try deactivating other plugins on the site and see if that corrects the problem, maybe you can narrow down what you’re looking for.
Is the gallery field being used to create a gallery in some other plugin that has been updated? Maybe that plugin’s requirements have changed for what needs to be done in the code.
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