Late June, 2018, some sites using Twitter for authentication started failing. App config settings adjustments were required to clear the error.
> Error: Callback URL not approved for this client application. Approved callback URLs can be adjusted in your application settings
There had been an announcement that further callback validation would be applied in June. twitter 
Paul diagnosed the problem. matrix
bay.wiki.org callbacks properly configured at twitter.
For those with a similar issue, the problem was that twitter have made change - which was only notified to developers via a tweet. This change meant that the URL that twitter will redirect back to as part of the signin has to be configured, and the value passed by the application has to match that configuration. Previously it would all just work without those URLs being configured.
Looking at the support forums a lot of people have been caught out by how Twitter has handled the communication about this change.