wp.media undefined

Thanks in advance for the time and help.

I’m using Advanced Custom Fields front-end form with a previously existing theme (that I did not create). The form has an image gallery field and Google Map field — neither of which work because of the error: “TypeError: wp.media is undefined” (Clicking on “add image” produces no effect, when it should pull up the Media Uploader window — the Maps field is just blank).

I know this is a problem with my WordPress theme, because when I switch to TwentyFourteen theme, the front-end form works perfectly. The developer of ACF has been very helpful, but this is not a problem he can help with!

I tried disabling my functions.php file to see if any of the old customizations were causing the issue — no luck.

I deleted all my WordPress core files and reinstalled (no luck — though as I said this is not a problem with the default WordPress theme, even with all my plugins enabled).

So the QUESTION — where do I start looking for the cause of this issue?

Plugins used: ACF, Members, Theme My Login, WP Ultimate Search, WP All Import Pro, Featured Image Column.

Thanks again,
Enrico