Sounds good @guenni if you do find the root cause is WP core feel free to open the original Trac ticket.
Hello @guenni
*Sorry my response is in English as I don’t speak German.
I’ve just closed your #48947 ticket on the Core Trac as I wasn’t able to reproduce using WordPress 5.3/TwentyTen/Classic Editor plugin.
From your description you’ve enabled the Classic Editor on the front-end which would be done through a plugin or customization. If you’re using a plugin for this I would start my investigation there and possibly open a ticket within that plugin-specific support forum.
The WordPress core and TwentyTen theme which are covered by the Core Trac don’t enable front-end editing which was the reason to my closing and directing you to continue seeking assistance here or through the plugin-specific forum if you’ve used a plugin to enable the front-end editing.
All the best