TYPO3 Core

On running "Introduce URL parts ("slugs") to all existing pages" wizard in Upgrade wizard It fails with the following error after a long time:

Fatal error: Uncaught TYPO3\CMS\Core\Error\Http\PageNotFoundException: The requested page does not exist in /typo3_src-9.5.0/typo3/sysext/frontend/Classes/Controller/ErrorController.php:82 Stack trace: #0 /typo3_src-9.5.0/typo3/sysext/frontend/Classes/Middleware/PageResolver.php(108): TYPO3\CMS\Frontend\Controller\ErrorController->pageNotFoundAction(Object(TYPO3\CMS\Core\Http\ServerRequest), 'The requested p...', Array) #1 /typo3_src-9.5.0/typo3/sysext/core/Classes/Http/MiddlewareDispatcher.php(138): TYPO3\CMS\Frontend\Middleware\PageResolver->process(Object(TYPO3\CMS\Core\Http\ServerRequest), Object(class@anonymous)) #2 /typo3_src-9.5.0/typo3/sysext/redirects/Classes/Http/Middleware/RedirectHandler.php(70): class@anonymous->handle(Object(TYPO3\CMS\Core\Http\ServerRequest)) #3 /typo3_src-9.5.0/typo3/sysext/core/Classes/Http/MiddlewareDispatcher. in
/typo3_src-9.5.0/typo3/sysext/frontend/Classes/Controller/ErrorController.php on line 82

The wizard generates slugs for 90% pages , but it misses out the remaining.
I tried to debug the situation ,but couldnt catch the exact control flow of the wizard such that it ends with a pagenotfound error.