My users are unable to set featured images

Hi!

My users are unable to set a featured image for a product. Tyler managed to somehow get it to work the first time this issue appeared, but now it's back again, and I just can't find a way to fix it. Here's the chat session at which he helped me with this: https://premium.wpmudev.org/live-support-archives/?lid=4068&chat_id=396176

The page at which the featured image is added is this (testaccount12 is the username):
https://www.wheelofcommerce.com/testaccount12/store-admin/ . You'll also notice a ghost image at the panel when you try to set the featured image. Another thing that might be related to this whole issue is the fact that there's a little icon of a broken image being displayed right beneath the store admin menu at https://www.wheelofcommerce.com/testaccount12/store-admin/?mpfe_page=allproduct

I first thought this could be caused by https, but it happens even when https isn't forced. Michael ran a plugin conflict and found out that users can set up featured images when WPMU Private Messaging is deactivated, which I confirmed. But then again: Private Messaging was active before when users were able to set featured images for their products, so it's unlikely that the plugin is causing this, even though it seems to be somehow related to this whole mess. Another thing I've noticed is that the button to "Remove featured image", that's displayed right below the featured image, doesn't work with WPMU PM turned on or off.

As you'll probably need it: my login credentials (including those for a test account) were sent around 3 days ago through WPMU contact form, with Aicee's name at the subject line.

Thanks.