I'm having the same problem. Completely uninstalled and then reinstalled V6.7.0.19 on Windows 10. Worked ok on a three test files add to a small folder. Checked online and the test files were there. Came back to the desktop to try another folder and got stuck in the Preparing Filelist. Here's the log from this attempt after I stopped the backup...
[Start Time: 9/18/2017 9:53:43 AM]
[End Time: 9/18/2017 10:06:21 AM]
[Files considered for backup: 61869]
[Files already present in your account: 0]
[Files backed up now: 0, Size: 0.00 B]
[Unable to continue the backup operation. Reason : Operation cancelled by the user.]
It should have had about 40 new files to add but the log doesn't even show the online condition. This has been happening a lot, and my last two weekly scheduled backups have just looped in this state for hours overnight.
Addendum: Closed the desktop app and reopened and tried the same backup. Now it stalls when trying to determine the files already online. Log...
[Start Time: 9/18/2017 10:13:23 AM]
[End Time: 9/18/2017 10:18:12 AM]
[Files considered for backup: 61869]
[Files already present* in your account: 1000]
[Files backed up now: 0, Size: 0.00 B]
[Unable to continue the backup operation. Reason : Operation cancelled by the user.]