Sync problem

After upgrading my Dropbox to 1TB, syncing between my Mac and iPhone is working, but there is a problem on my iPad I do not understand. When I tap the cloud icon to start a sync, at first I got a message saying I had an invalid encryption key. I edited the sync location, found the place to enter the key, and did so.Saved it. I’m quite sure I have the right one.

Tapping the cloud icon again, I get this message: “That actioni is not authorized. Please ensure that you are authenticated.”

Huh?

Checking the cloud icon, it seems to be processing, and tapping it shows it in the process of downloading a bunch of stuff. So, what was that bit about authentication?