The guest user is a user that is not logged in, so Laravel did not log you in correctly. Really need more detail here, as if you are using the default routes and default login code, and authenticating against email/password then there's not a lot that can go wrong.
Try debug tracing through the method calls where it performs auth->attempt, check what credentials are being sent and compare and what the outcome of that method is. Make sure your login page dumps the $errors so you can see if there are any coming back.
Only thing that jumps out at me is you have added in an attribute setter for the password field. I'm wondering if you are potentially double hashing your password which is then causing the failed logins, as the bcrypt already happens as part of the default Laravel user registration.