Can't load image to Site Categories

Hello,

I can't seem to add an image to a category in the Site Categories plugin. This happens on two of my sites.
chinook.school-core.com and
e-core.ca

I've deactivated all plugins and used the Twenty-Sixteen theme. I click the add image button and nothing happens. I saw that there was a thread about this in the past but there was no solution posted.

https://premium.wpmudev.org/forums/topic/unable-to-ad-an-image-in-site-categories

Thanks,

jayjay