TYPO3 >= 9.5.12 - Cannot use object of type __PHP_Incomplete_Class as array

[Bernhard Berger]

On TYPO3 9.5.12 and later this commit breaks the session entities on validation errors:

if (isset($this->internalArguments['__referrer']['arguments'])) {
                // This case is kept for compatibility in 7.6 and 6.2, but will be removed in 8
                $arguments = unserialize(base64_decode($this->hashService->validateAndStripHmac($this->internalArguments['__referrer']['arguments'])), ['allowed_classes' => false]);
            }

https://github.com/TYPO3/TYPO3.CMS/commit/b1626ad8fd#diff-f79bd288f1dd87d7748276181e71928cR194

The problem manifests in EXT:extbase_session_entities/Classes/Mvc/PropertyMapper.php:30:

public function convert($source, $targetType, PropertyMappingConfigurationInterface $configuration = null)
    {
        if (is_subclass_of($targetType, AbstractSessionEntity::class) &&
            isset($source['__identity'])
        ) {
            $configuration->skipUnknownProperties();
        }
        return parent::convert($source, $targetType, $configuration);
    }

Result: Cannot use object of type __PHP_Incomplete_Class as array

Any ideas on how to fix it?