The NullPointerException is just informational for the “Received invalid iCalendar, ignoring” line, so everything except this event should have been synchronized.
ical4j throws this exception when trying to parse the VTIMEZONE because there’s an invalid component in your VTIMEZONE definition:BEGIN:DAYLIGHT TZOFFSETFROM:+0119 TZOFFSETTO:+0119 TZNAME:NST END:DAYLIGHT
Time zone components must have a DTSTART value, otherwise they’re useless and invalid. This is similar to DAVdroid doesn’t understand America/Montevideo VTIMEZONE provided by Horde and Can’t see calendar events.
Because the time zone is not parseable, ical4j and thus ical4android/DAVdroid cannot process the event. To fix it, please use a correct time zone definition.
I also suggest reporting this to Horde.