Insert Image – Media Library not loading

Hi there,
I’ve been working on a new site for Inverness Mountaineering Club, and everything seemed to be going fine until today.
I started a new post for the site, and when I went to Insert Image, the Media Library frame loaded, but sat there with the spinning circle and did not display any images, regardless of selecting ‘Images’ or ‘Uploaded to this post’, and if I uploaded an image, I’d receive the error ‘An error occurred in the upload. Please try again later.’
Avia is enabled for posts, as per the link in the Feature Request forum, but I have tried removing the code from functions.php and still encounter the same issue with Add Media in the standard post editor. The Insert Image also fails to load the Media Library in Pages.
If I got to Media, all uploaded images, including the ones which I receive the error message for, appear without issue.
The reason I’m posting here is, if I switch to the Twenty Fourteen theme, the Media Library works again, it only fails with Enfold enabled.
I’ve enabled WP_DEBUG, but nothing at all relevent appeared in debug.log. I ran a reinstall of WP 4.0.1, but issue remained.
Any suggestions as to what the issue may be or where I should look?

Many thanks,
–Richard