Cannot attach images to gallery

Hello,

I'm using this module on Drupal 7 but have recently started getting an error when trying to upload images, I'm unsure as to what was done to cause it as a fair amount of theme changes have been made before I tried using this again.

When I look in the developer tools console after trying to upload an images to a gallery I get a "http://domain.com/media-gallery/add-images/5302/l0YjdLmRjXj2ky1mYrvRX1cu... 500 (Service unavailable (with message))" error.

Does anyone have any idea on where to look for debugging this? I'm using the latest versions of file entities, media & media gallery modules and the images are being uploaded to Amazon S3, I've checked and the images are actually uploaded, they're just not attaching to the gallery.