Laravel error - Class setEventDispatcher does not exist

Short version

I'm getting this error, when I run php artisan serve and visit my home page:

ReflectionException (-1)
Class setEventDispatcher does not exist

Details

Laravel version: 5.6.39 Environment: local OS: macOS Mojave 10.14.2

It started, when I got an error using npm (in another project), so I reinstalled Node (this way).

After doing that, then this project (in question) couldn't be started. I was getting this error, when running php artisan serve:

dyld: Library not loaded: /usr/local/lib/libpng16.16.dylib with anything php related
Referenced from: /usr/local/bin/php
Reason: image not found

That error was resolved by updating and upgrading brew.

Then I was capable of starting the project (php artisan serve). But when I visit the home page, this error was shown:

ReflectionException (-1)
Class setEventDispatcher does not exist

If I follow the stack trace, then I can see that it was this line (from my homeController) that triggered the error:

if( ! Auth::check() ){
  return view( 'pages.home' );
}

If I commented that out, then the same error occured from another place, where the Auth-module was being used.

Here's the stack trace:

Does anyone know how to resolve this? I can't find any promising results on Google, - and I have no idea why this happens... :-/

Addition1

I tried composer clear-cache and deleting the vendor-folder, and then doing a composer install and the problem still occured.

I also checked my Service Providers, as Marcus suggested, - but I don't really know what to look for in there. It all looks pretty regular (no fancy if-statements or anything).

Addition2

I realized, that I couldn't connect to my database using Sequel Pro. And it was because Brew had updated my MySQL-version from 5.5 to 8.0.

That was resolved by doing something along these lines (I did a bunch of stuff, so I might be missing a coulple of steps):

brew unlink mysql
brew install mysql@5.7
brew switch mysql 5.7.21
brew link mysql

and I could then verify the version with mysql --version. I also had some permission issues with that MySQL-version-change, but I'm emitting that, so this doesn't get too long.

Addition3

I tried disabling the Laravel Debugbar (by deleting it from composer.json, deleting composer.lock and vendor and then running composer install). And that fixed it!! ... But!

When I later tried to install Barryvdh's IDE Helper, then the exact same error came back!?

I performed these three steps, to install the IDE Helper (and re-introduce the error):

1. Run this command from the root of my directory: curl https://gist.githubusercontent.com/barryvdh/5227822/raw/4be028a27c4ec782965bb8f2fdcb4c08c71a441d/_ide_helper.php -o _ide_helper.php

2. Add these lines to app/Provider/AppServiceProvider.php:

...

if ($this->app->environment() !== 'production') {
  $this->app->register(\Barryvdh\LaravelIdeHelper\IdeHelperServiceProvider::class);
}

3. Run this command: composer require --dev barryvdh/laravel-ide-helper

So I'm back at this error:

ReflectionException (-1)
Class setEventDispatcher does not exist

... The stack trace is identical to the one shown previously (sprung from Container.php).