The app references non-public selectors authenticationDidCompleteWithError:

Suddenly getting this error after making minor (storyboard) updates to app, when trying to upload to App Store connect.

App uses Firebase and RealmSwift API's, so I assume one of them is referencing the API in question. However, I have not updated either framework for months, and previous build was accepted and approved by apple.

Searching forums, it appears similar problems were occurring for developers about 3 moths ago. Apple declared it a problem with backend and "fixed." Appears to possibly not be fixed any more...

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I am also seeing this issue. The app has RealmSwift but not Firebase

See this Github issue thread: https://github.com/realm/realm-swift/issues/7945

This warning can be ignored if you need to stay on the older version of Realm (allegedly will get past app review). Or you can update to the latest version of Realm (10.30.0 fixes this issue for me).