I have same issue: few months ago i setup the website with the gallery, posted content and all was ok.
today i’m adding a new page, and in the metabox page there’s not the “browse”to add images, and in the ACF setting now the Gallery post type is “unknown”.
How did you solved?
Last week, we released ACF 5.10 🎉 This update includes the brand new HTML escaping feature, which helps to keep your ACF-generated content safe. Learn about this and other new goodies in the release post: https://t.co/p9gKUYmdDQ