Font awesome icons not showing: 'access-control-allow-origin

I recently switched to a multisite, but since then the font awesome icons don't show up anymore on one subsite of the multisite. The icons do show up on my other subsite. On my staging site, also a multisite, the icons show up on both sites.

Access to font at 'https://secureservercdn.net/blabla.com/wp-
content/themes/ave/assets/vendors/font-awesome/fonts/fontawesome-webfont.ttf?v=4.7.0' 
from origin 'https://example.com' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource.

What could be the problem?

edit: investigating further reveals that not only font awesome yields such a error but also another icon package provided by my theme.