I'm facing same issue. Watching closely, I noticed that the header that is being passed contains not just a newline char, but also html code. A code for a page just annoucing the redirection to the route I wanted it to redirect.

My login is redirectTo is

/**
 * Where to redirect users after login.
 *
 * @return string
 */
protected function redirectTo()
{
    return redirect()->route((string)\Auth::user()->group->type);
}

Where already exists a named route for each group->type possible.

I've already tried just a return to the route path with no success or any different error message.

[UPDATE] I found a fix here, changed my code to this and it worked:

protected function redirectTo()
{
    // return redirect()->route((string)\Auth::user()->group->type);
    return route(\Auth::user()->group->type);
}

you should use

return route('admin.dashboard');

instead of

 return redirect('admin/dashboard');

Try it in LoginController.

public function redirectTo(){

   $user_id = Auth::user()->id;

   $role_manager = User::where('id', $user_id)->value('role');

   if($role_manager == 'manager'){

       return $this->redirectTo = 'dashboard';

   }
}