Thanks Halftux for the suggestion. Someone else in this thread mentioned something like that, that they copied the link from Rocket, but the internal browser does not show the link, so maybe that was for signing up rather than signing in. Also, Opera is my default browser, and when I tap "privacy policy" rather than "sign in " on the Rocket welcome page when it launches, then instead of the internal browser, Opera launches (to show me a Twitter page, the privacy policy). Likewise, if I tap the "sign up" button, that opens a Twitter site in Opera (and I'm told that my browser is no longer supported). In other words the sign in button only allows launching the internal browser and does not show me what this link is, even though the other buttons launch Opera from which I can copy links.

Now I'm back to being signed in like before but everything is empty, like before; no feed, no search results, no lists, and on my account page, no info about me (my name and username fields are empty), and when I add the info and tap "update," it is still empty when I come back to it. But there is a tappable Sign Out button, so I'm sort of signed in, maybe. Plus now when I launch Rocket, it no longer shows me the sign in/sign up page but instead, the empty feed. I bet there is something I could copy over from my working Rocket account on my other N9 and then it would work, since the app is authorized for my account but not really logged in. But it thinks it's logged in. I'd think it just no longer can work but again it is working on my other N9 that did not get the backup-restore procedure, something I had expected to be non-destructive.

Rocket is still available on Warehouse (5k downloads, 15 upvotes, 5-star app....). So I was thinking I had a unique problem because it broke upon a backup/restore data step. But the last comment for Rocket on Warehouse is from over 2 years ago (and it happens to be mine!). Very sad because it is a nice app.