LoginView with PolymerTemplate not recognizing error parameter

I have a LoginView which I basically copied from the bakery starter project.

When I run the bakery app and try to login with wrong credentials, there appears an additional div with an error message.

However, this error message div does not appear in my view. I have since adapted the template to our Corporate Design, but I can remember that this issue already existed when the two files LoginView.java and login-view.html were still exact copies from the bakery app. I double checked that .error is not just hidden by my own css - the div.error is not existing in the DOM.

Debugging the LoginView when visiting /login?error (visiting login?error=true is the same) shows that it does not recognize any queryParameters! → Why?

[My App debugging screenshot]
(https://imgur.com/z6JeaTw)

[Bakery App debugging screenshot]
(https://imgur.com/fzKh3Df)