Hi @Benjamin Maywald ,
According to the official article below, looks like what you are experiencing is a known issue still under investigation:
The new Outlook for Mac - Known issues
Given this situaiton, it's suggested to swtich back as you have already done. And meanwhile, we can check the article above periodly to see if there would be any update on this issue. I'll also keep an eye on it and post back here once I see any new information.
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Update ( 6/23/2021:)
According to the latest admin response in the Outlook UserVoice forum, copy/move messages between two accounts is now available in version 16.50(21061301) and newer:
Not able to drag/move e-mails from one mailbox to another anymore.
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