Using Keynote as virtual background failing - "Keynote not installed" message

I have been using the zoom feature on the "slides as virtual background" screen share for a long time. So far, this has worked flawlessly on my Macbook with Keynote. Since a short time it does not work anymore. The following message is displayed.
Keynote works fine on my MacBook.
Who can help?

Sorry, my attachment is in German. It means "Import from "test.pptx" is not possible because keynote is not installed"