Single view of December events results in "page not found"

All December events result in "page not found" in single view. This happens with events in Dezember 2017 and December 2018. Events in November 2017 or January 2018 work fine.

Examples:
error (shows default content): https://tegel.kiezekatze.berlin/events/2017/12/lesezeichen-literatur-live-in-tegel-5/

no error: https://tegel.kiezekatze.berlin/events/2017/11/konzert-im-stile-der-wiener-klassik/

Wordpresss was just updated to latest release.