In a site page gallery settings, under media, the 'ADD IMAGE' button can't be clicked to add a thumbnail to the page

Hi.
I'm trying to update some thumbnail images to my portfolio site which are currently not showing (the webpage currently shows a missing image link instead of the image or GIF that used to be there) this usually works fine, I've dozens of pages live for several years, but now If I navigate to Website > Pages >  choose the cog icon on one of the pages which pops open the vertical list of gallery settings for that page (general, SEO, social image, media, advanced), when I go to 'media' and to 'add image', I'm unable to click the 'add image' button, just nothing happens, no error message, no spinning wheel, it's just unclickable. I have 4-5 instances of this across my portfolio homepage, as per image attached.

Any thoughts? Many thanks.