@IDH: Thanks for this information. I've just updated Outlook to version 16.58 and the search function is working again; what a relief! Can others confirm that the search issue is gone now with this version...?

I would really like to switch to the new outlook GUI to have the search available again, but when I click on the switch, a message with my mail accounts appears telling me that these cannot be used with the new Outlook (see screenshot: https://wolf.d.pr/n6rq3U ). Since I really need all these accounts (most of them POP) including all the actual mails I didn't dare yet to make that switch. I fear that I won't have access to all the mails anymore. Maybe someone here can shed some light on this...