In Muse on Win 10. Here's what I get when I click on "Add Web Fonts". I'm on a subscription with all apps except Stock and everything is up to date. The Edge Webfont Tab works fine, but Typekit is not loading. I've stopped and restarted Muse several times, but no difference.
I found an earlier entry in the Forum (2015) which speaks of stopping "CC Libraries" in the Task Manager, but I couldn't find anything of that name.
Below is the window I get when I try to open Typekit.
I have no idea why it worked, or how it happened in the first place, but I restarted the PC (which I don't do every day) and the previews are back.
It could be related to an attempt I was making before the problem first appeared to install self-hosted Webfonts for Muse - an attempt that failed - which is the point I went to TypeKit and discovered the previews were gone. Anyway, if this happens to anyone, you should FIRST try restarting the PC, not just Muse.