It started with this update beginning around 14 November. The "old" 404s and the page they are extracted from have been gone for around a year. But now they are back.
It corresponds with some posts about google bot started to fetch pages like crazy. Seems like the index has to be updated/proofed maybe for some recalculation.
But it is annoying if you do the best to keep your site updated/clean an google canīt catch up with your changes.