Hi again,
I seem to have solved my problem on my own. I had installed the Yoast plugin a few months ago. This may have caused problems, but I can’t say for sure. I deactivated it today and then recreated all the events created in June. Only the editing did not work. The events were only displayed after they were created again. In the end, even though I reactivated Yoast. Very strange, but I’m just glad that all events are displayed again.
PS: My wordpress is up to date as are all my plugins. PHP version on my server is 8.+