It looks like the Let's Encrypt Certificate for assets.fanart.tv expired nearly two months ago, which is preventing apps like Emby from downloading FanArt.
For example, in reply to:
https://assets.fanart.tv/fanart/music/6 ... ec998f.jpg
I get:
assets.fanart.tv uses an invalid security certificate.
The certificate expired on 10 October 2018, 10:25:24. The current time is 7 December 2018, 10:20.
Error code: SEC_ERROR_EXPIRED_CERTIFICATE