I found the cause of the problem.
In Okta it is optional to allow the issuing of Refresh tokens.
We had not enabled this. As soon as we enabled it, everything works.
In the code, the Refresh token is assumed to be available after a successful login request, and there’s no check before it is assigned to a variable, which causes the null reference.
Issue in source code is in LoginAsync in
Engine\Source\Programs\Shared\EpicGames.OIDC\OidcTokenManager.cs
_refreshToken = loginResult.RefreshToken;