Thanks for your recommendation, @prologic@twtxt.net, I ended up going with it. Due to a parser panic I found a bug in one of my own iCalendar generators. The start date was announced as a DATE only, but in fact a complete timestamp including time and timezone was written instead (DTSTART;VALUE=DATE:20230106T000000Z rather than DTSTART;VALUE=DATE:20230106). This parser didn’t like that. To my defense, the validator did not report anything and approved of my ical file, though.