Digging into the issue together with @jensfk it turned out that for a reason beyond our comprehension a checkbox in the polylang plugin settings was disabled, with the effect that podcast episodes where no longer considered to be multilanguage, thus no longer saved with a language indicator upon creation or update. Podlove, when building the feed and querying for new episodes with a specific (German in our case) language key tagged to them, did not find newly created ones without that key, so the did not make it into the feed.
We had already noticed that the WP admin episodes editor had no longer a language widget/dropdown in the right pane; the eventual clue came from RSS feed not working together with polylang · Issue #1090 · podlove/podlove-publisher · GitHub . The solution has indeed already been mentioned here in the podlove community forum, thougly less explicitly so: Doc/solution: Podlove publisher and polylang (18n)