Symfony 4 - ClassNotFoundException Kernel

Symfony 4 uses the folder App for autoload psr-4. I tried to change it, but it didn't work out. Check the namespace at your composer.json file, in the property autoload and then psr-4. Maybe you changed the default one.

Maybe you just accidentially removed the PSR-4 block in composer.json that is always needed for Symfony4:

"autoload": {
    "psr-4": {
        "App\\": "src/"
    }
},