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Every time I click "Save" or "Clear cache" on my prstashp back office advance settings menu, appear a white page with "HTTP ERROR 500" and I cannot use my backoffice until I restore previus file backup. The server support say me that the log of the error is:

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[29-Nov-2019 10:29:38 Europe/Rome] PHP Fatal error: Uncaught Symfony\Component\DependencyInjection\Exception\RuntimeException: Cannot dump definition because of invalid class name (NULL) in /home/architet/jacobuscalzature.it/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php:1403
Stack trace:
#0 /home/architet/jacobuscalzature.it/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php(790): Symfony\Component\DependencyInjection\Dumper\PhpDumper->dumpLiteralClass('NULL')
#1 /home/architet/jacobuscalzature.it/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 /home/architet/jacobuscalzature.it/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php(639): Symfony\Component\DependencyInjection\Dumper\PhpDumper->addServiceInstance('_defaults', Object(Symfon in /home/architet/jacobuscalzature.it/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php on line 1403

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Why appen this problme andhow can I solve this ?