Error: While updating laravel 8 to 9. Script @php artisan package:discover –ansi handling the post-autoload-dump event returned with error code 1

The colleague with the answer above is, in principle, right. Only he forgot to mention that after all the changes in the file, you need to delete the package fideloper/proxy:

https://laravel.com/docs/9.x/upgrade

Trusted Proxies

Likelihood Of Impact: Low

If you are upgrading your Laravel 8 project to Laravel 9 by importing your existing application code into a totally new Laravel 9 application skeleton, you may need to update your application’s “trusted proxy” middleware.

Within your app/Http/Middleware/TrustProxies.php file, update use Fideloper\Proxy\TrustProxies as Middleware to use Illuminate\Http\Middleware\TrustProxies as Middleware.

Next, within app/Http/Middleware/TrustProxies.php, you should update the $headers property definition:

// Before...
protected $headers = Request::HEADER_X_FORWARDED_ALL;

// After...
protected $headers =
   Request::HEADER_X_FORWARDED_FOR |
   Request::HEADER_X_FORWARDED_HOST |
   Request::HEADER_X_FORWARDED_PORT |
   Request::HEADER_X_FORWARDED_PROTO |
   Request::HEADER_X_FORWARDED_AWS_ELB;

Finally, you can remove the fideloper/proxy Composer dependency from your application:

composer remove fideloper/proxy