TYPO3 Core

In 9.5.0 that has already worked, but no longer in 9.5.1. The TCA configuration is the following:

'slug' => [
    'exclude' => 1,
    'label' => 'LLL:EXT:core/Resources/Private/Language/locallang_tca.xlf:pages.slug',
    'config' => [
        'type' => 'slug',
        'size' => 50,
        'generatorOptions' => [
            'fields' => ['title'],
            'fieldSeparator' => '/'
        ],
        'fallbackCharacter' => '-',
        'eval' => 'uniqueInSite',
        'default' => ''
    ]
],

The creation button in TCE form doesn't work (nothing happens) as well as entering a slug manually leads to the following error when saving the record.

Core: Exception handler (WEB): Uncaught TYPO3 Exception: Argument 1 passed to 
TYPO3\CMS\Core\DataHandling\Model\RecordStateFactory::
TYPO3\CMS\Core\DataHandling\Model\{closure}() must be of the type string, null given 
| TypeError thrown in file /www/htdocs/_TYPO3_/typo3_src-9.5.1/typo3/sysext/core/Classes
/DataHandling/Model/RecordStateFactory.php in line 103