> If you create a new account on en.wiki and then try to log in to an OAuth app that is authenticates on en.wiki (for example, http://wizard.wikiedu.org), you will get that same error ["Unified login needed, E008"] every time until you visit another wiki.
The central OAuth wiki (for now, mediawiki.org) is auto-attached to the global account the first time the user visits a wiki where he does not have a local account. That never happens if the user registers on the same wiki where the app authenticates (as long as that wiki is not the central one).
See also T74469 and T104932
UPDATE: This appears to be fixed for new users, but can still affect users who created their accounts before the other related fixes for this task.