Using older versions of Outlook for Mac for personal email and calendar
Hi Christian,
Thank you for posting your issue on Microsoft Community. There are several features that aren't yet available in the new Outlook experience and some known issues such as Mail and events can't be moved or copied between accounts. However, our product team is investigating this issue in the new Outlook for Mac. You may switch back by turning off the New Outlook switch to avoid this issue.
For detailed information see under Features not yet available in the new Outlook for Mac/Known issues
Your time and understanding will be highly appreciated.
Best regards,
Shyamal