Like many of you, I have experienced the growing pains of transition to Fab.com. My entire library disappeared from the Epic Launcher, Fab UE Plugin was nowhere to be found, etc. I had the most recent version of the launcher and the engine installed and just could not get it to work.
What solved it for me was enabling game library for a moment. Unchecking the box next to Hide Game Library somehow “unplugged” the launcher and made it connect to Fab and pull the data.
I am sure this is a bug and not the intended behavior - but to all that still struggle, try it out!