Hi Predrag @wpmudev-support1,
Thank you for replying.
I followed your advice and I think I have found the problem.
The website is one that I recently took over from another agency. The previous agency used WPMU DEV’s premium plugins and were utilising the WPMU DEV dashboard plugin.
I don’t have a WPMU DEV membership (yet). So the dashboard and premium plugins were pretty much useless for me. By disabling all plugins, and re-enabling them one-by-one, I found the when I re-enabled WP Smush Pro, it caused the issue I was having to reoccur.
I can now resolve this issue. Thanks for your help.