For some reason an account which I've registered using my app and tested before is now giving me an error when trying to log in using its details: "Logins don't match. Please include at least one valid login for this identity or identity pool."
I know the values I am using for it are definitely correct and other accounts are still able to login. What could be the cause of this error? Any help would be greatly appreciated.
What the SDK is trying to tell you is that the identityId in your Federated Identity Pool (and which may be stored in a keychain in your device, and which may be re-established on restart from an existing session) does not match the login in your authentication provider which was returned by the "logins" method.
This can happen in a number of different ways, but this usually occurs because you attempt to log in as another user on the same authentication provider without first logging out.
The SDK recovers by retrying.
(This recovery does not really work because I think it takes 2 or 3 attempts to time out, and the recovery then leaves you in a state where subsequent logins fail with the same error. Restarting the app clears this. I have not fully investigated the defect in the retry/recovery process)
The solution is to prevent the app from logging in on a different ID with the same authentication provider (IdP) without first logging out of that authentication provider.