I have this problem on 4 windows 10 machines.
I had a missing sync drive (716GB), all data including the online view just vanished (it did remain locally, but not visible to sync folders). After about a week and a few tickets it eventually showed back up, but no update.
I have also experienced the infinite preparing backup list loop as described on multiple pc's.
The product has periodically stopped working and the answer has always to update to a new version. I am used to that but it is annoying, why should a x.x.x.x version update be the difference between working and not working?
I have sent multiple logs on the failure to backup / loop / no files message. I then get a ticket# again referencing the first ticket# that I sent the log from asking me to send the log which triggers the same request again!
At one point a reply came asking to view my PC and a time was picked, I agreed, but never heard back as how to do that.
It seems something really went wrong around the 6.8.1.1 update.
Is anyone correlating all of the same issues from multiple geographically disperse users? Seems something is missing in the support process.