Media Library thumbnails not showing up

I'm running a WPMU Network, with the latest version of Wordpress. For some reason my media library images are not showing up any more (although they are fine in the public websites) -- just a gray box with a missing image symbol.

I also notice that the url for each images is domain/files/year/month/image-name, instead of something like wp-content/uploads/sites/33/2017/03/image-name.

Editors are complaining that they can't easily find the image the are looking for in the Media Library.

How can I fix this??