TYPO3 Core

@Kurt Kunig: Short question, are other extensions like news installed?

The error is probably in the following line:
'displayCond' => 'USER:' . \TYPO3\CMS\Core\Compatibility\PseudoSiteTcaDisplayCondition::class . '->isInPseudoSite:pages:false',
in ext/news/Configuration/TCA/Overrides/sys_category.php

if (version_compare(TYPO3_branch, '9.5', '>=')) {
    $newSysCategoryColumns['slug'] = [
        'exclude' => true,
        'label' => 'LLL:EXT:core/Resources/Private/Language/locallang_tca.xlf:pages.slug',
        //'displayCond' => 'USER:' . \TYPO3\CMS\Core\Compatibility\PseudoSiteTcaDisplayCondition::class . '->isInPseudoSite:pages:false',
        'config' => [
            'type' => 'slug',
            'size' => 50,
            'generatorOptions' => [
                'fields' => ['title'],
                'replacements' => [
                    '/' => '-'
                ],
            ],
            'fallbackCharacter' => '-',
            'eval' => 'uniqueInSite',
            'default' => ''
        ]
    ];
}

The best solution would be to create an overwrite for the TCA in its own extension:

YOUR EXT:/Configuration/TCA/Overrides/sys_category.php

<?php
defined('TYPO3_MODE') || die();

$tmp_columns = [

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

];

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTCAcolumns(
    'sys_category',
    $tmp_columns
);