Unexpected end of JSON input

When registering a new account on my site, once I select an avatar and cover photo and click next, I am presented with a white page. It appears the account is created but this should not happen. When I refresh the page, I can login with the account I just created but the avatar and cover images were not saved. When trying to upload a new avatar or cover photo, I get an "Unexpected end of JSON input" error (image attached).

site address: BarFactory.com