Hi @Loibl, Nadine,
Per my test, I am unable to reproduce your issue. In my site, I can add event webpart.
You can take following steps to check
- Clear browser cache or use the other browser.
- Try to create the webpart in other page or site.
- Check if other users have the same issue.
- If you have custom code in your page, clear them and try to add the web part.
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