Hi everyone
Thank you for your replies.
@Liggie: The downgrade to the version you mentioned did only resolve the issue with the backend editor, but the save button in the frontend does still not work.
@Saba: The setting was already set to Legacy Mode. I changed it to Auto/Newest but it did not influence the behavior. I already tested various settings of Core Legacy and Legacy Routing Mode without success. Currently following settings are active:

I still don't understand why I am able to create a new article via frontend, but am not able to edit (and store) the changes.