W3 Total Cache and Azure CDN setup

Question

Setup:

1. W3 Total Cache WordPress plugin installed
2. Azure CDN activated via W3 Total Cache
3. All media and theme files synced with CDN
4. After CDN activation all works fine for ~30min
5. After ~30 min I got an mixed content error for all my website fonts and website looks broken
6. After clearing the cache from the server (not from the W3 Total Cache) it works fine again.

What it can be? How to fix this weird issue? Why the CDN redirects to the http connection suddenly? Thanks