Thank you for describing your steps! In this case, it really looks like the username you entered doesn't exist.
It will be the same if you go to e6 and type "favoritedby:username" in the searchbar. If this does not give you your favorites, neither will the program.
For example, "lopunny" is a username that exists and has favorites. Going to e6 and entering "favoritedby:lopunny" will give you the expected results, and same in the program if you type the username "lopunny" in e621 favorites.
I am working on a way to somehow tell if a username exists, I would suggest waiting for the new version that will hopefully be published in a few hours from now!