Problema accesso backoffice

[nicolman77]

Buongiorno, è da giorni ormai che sono disperato.

Ho la versione Prestashop 1.7.3, dopo l'aggiornamento ad una versione PHP più recente e all'installazione di un tema mi sono ritrovato a non poter più accedere al backend, il sito lato front funziona bene, ma lato backend mi da questo errore:
Fatal error: Uncaught Symfony\Component\DependencyInjection\Exception\RuntimeException: Cannot dump definition because of invalid class name (NULL) in /homepages/17/d741200733/htdocs/clickandbuilds/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php:1403 Stack trace: #0 /homepages/17/d741200733/htdocs/clickandbuilds/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php(790): Symfony\Component\DependencyInjection\Dumper\PhpDumper->dumpLiteralClass('NULL') #1 /homepages/17/d741200733/htdocs/clickandbuilds/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php(394): Symfony\Component\DependencyInjection\Dumper\PhpDumper->addNewInstance('_defaults', Object(Symfony\Component\DependencyInjection\Definition), 'return ', '$this->services...') #2 /homepages/17/d741200733/htdocs/clickandbuilds/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php(639): Symfony\Component\DependencyInjection\Dumper\PhpDumpe in /homepages/17/d741200733/htdocs/clickandbuilds/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php on line 1403

Vi prego di aiutarmi per favore.

Grazie