Hi @Christoph Fink ,
By "Outlook Online", are you referring to Outlook on the web accessed via https://outlook.office.com/ using an M365 account?
I just tried testing in my lab tenant and noticed that I can only move the event rather than "copy" when I use Mouse + CTRL in OWA. Is this what you are experiencing? If this is the case, seems currently there's no settings available in OWA to change the behavior.
Given this, if possible, you can switch to Outlook desktop client where the Mouse+CTRL can still be used to copy the event:
Meanwhile, I've just tried sending a feedback via the Feedback button. Hope this can be changed in the future.
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