Username-Password Log in with Lock does not work

Note: The related question
did not solve my issue.

I’ve set up my Swift App with the Auth0 Web Login and everything works fine. Then i’ve tried to implement the “Lock” login and social media logins work perfectly well, but i cannot login via the username-password method. On my Auth0 database, the signed up user shows up and testing on the Auth0 HP works fine as well. Just when trying to login with the Lock widget, i get an error: “We’re sorry, something went wrong when attempting to log in.”. I also tried verifying my E-Mail, but that did not solve the issue either.
Any ideas, what might go wrong here?