"Swap Image" error with Upfront

With a fresh WP install, no plugins installed besides the WPMU Dashboard, and install of Upfront theme (tried with a couple different child versions), but can't change or add any images. With the ordinary "Swap Image", it's replaced by text instead of button, and clicking on it simply freezes the page, as pictured. I've tried two separate fresh installs. Additionally, tried on another existing text site, and it worked as desired, with no rhyme or reason as to why. Tried cloning that test site, and had the same errors as before.