Using Classic Outlook on Windows for personal email, calendar, and contact management
Dear Admins,
you solved the problem within one week after this post. Thank you very much!
Now, there is a checkbox in the settings which allows disabling the deleting of invite messages.
Navigate to View > View settings > Calendar > Events and invitations and unselect "Delete invitations from inbox after responding" in section "Invitations from other people".
Kind Regards
Micha