Ads won't post or edit

I've had the classifieds plugin installed for over a month and things have been running relatively smoothly. However, recently my users stopped being able to choose categories for their ads (the list is there still, but they cannot check the boxes), and recently I've found that even as the admin, I am unable to add the categories for them. I can check the boxes, but when I hit "update", a red box appears above the featured image saying that the featured image field is required. There is a featured image present, so I'm not sure what the problem is, but the ad will never update.

Any help will be greatly appreciated!