I had what looked like a similar problem starting at o/a 20 Feb 2018. My primary symptom was the app was able to sign in but started having errors in during the sync process. It finally got stuck in the downloading/update part of the process. I let it sit a couple of times for 2 or 3 hours, but only had the spinning circle with no visible progress.
I reset the app and selected the option to download 15 common books. Got the spinning wheel again. I gave up after a 20 minute wait with it still saying there was15 items left. I have a really, really fast internet connection. 20 minutes is normally long enough to download multiple gigabytes.
I uninstalled and reinstalled the app (version 4.1.24) and when through the same setup questions. This time it only offered to download 12 items. I gave it about 30 minutes while I was doing something else. When I got back to my computer, I saw a Launch button, clicked on it, and it began working. I then manually downloaded a number of favorite items and it has been working fine for the last hour or so.