TYPO3 relies on HTTP referrers for security reasons in the backend. It seems your browser refused to send this information.
More about TYPO3
TYPO3 CMS. Copyright © 1998-2021 Kasper Skårhøj. Extensions are copyright of their respective owners. Go to https://typo3.org/ for details. TYPO3 CMS comes with ABSOLUTELY NO WARRANTY; click for details. This is free software, and you are welcome to redistribute it under certain conditions; click for details. Obstructing the appearance of this notice is prohibited by law.
Arbeitskreis Ostertalbahn (AkO) e.V.