[Smush Pro] Throwing 503 for images

We have a problem on our site with Smush CDN. The images are not showing up and they return a 503 error. If you open the direct address of any image, they return the following error:

Error 503 Backend fetch failed

Backend fetch failed
Guru Meditation:

XID: 554206404

Varnish cache server

The site is hosted with WPMU DEV hosting and we don’t have any caching plugins on the site. Please advise.