Laravel 5.5 Login errors not showing up

Try to remove Session::get('errors') from your if statement in login.blade.php

@if(count( $errors ) > 0)
    @foreach ($errors->all() as $error)
       <h1>{{ $error }}</h1>
    @endforeach
@endif

ShareErrorsFromSession middleware, which is provided by the web middleware group is responsible for $error view variable so it will always be defined (link here)

[UPDATE]

And as @Ohgodwhy pointed, you need to use @if ($errors->any()) Example

So in your case it will be:

@if($errors->any())
    @foreach ($errors->all() as $error)
       <h1>{{ $error }}</h1>
    @endforeach
@endif

Ok, after a few hours I finally found it! I created a Laravel project from scratch and made a diff to find the culprit:

In app/Http/Kernel.php, make sure to get rid of the StartSession middleware:

protected $middleware = [
    \Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
    \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
    \App\Http\Middleware\TrimStrings::class,
    \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
    \App\Http\Middleware\TrustProxies::class,
    \Illuminate\Session\Middleware\StartSession::class, // <-- Remove this
];

Explanation: I had it there because I read that I had to put it as a middleware (if I wasn't using the Route::group(['middleware' =>'web'] wrapper in my web.php), I think that I forgot it there. I think that putting it there and using the wrapper in web.php somehow truncate the error session before it gets to the view.

Put,

Auth::routes();

Inside middleware group.

Web middleware starts the session. If you are writing any route outside that middleware group then you can not access the session.