told the airport is closed ( due to it being midnight )
whatever the reason was, the time it was probably not, because while cancellations are rare, the flight in question took off from WAW several times this year after midnight, in particular the day before 7 38 min. past midnight:
DateAircraftOriginDestinationDepartureArrivalDuration
25 cze 2019DH8DWarsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)12:30AM CEST01:08AM CEST0:37
20 cze 2019E75LWarsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)12:08AM CEST12:45AM CEST0:37
14 cze 2019DH8DWarsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)12:08AM CEST01:05AM CEST0:57
7 cze 2019DH8DWarsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)11:00PM CEST11:40PM CESTCancelled
6 cze 2019E190Warsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)12:38AM CEST01:14AM CEST0:35
4 kwi 2019B734Warsaw Frederic Chopin (WAW / EPWA)Copernicus Wrocław (WRO / EPWR)12:14AM CEST12:53AM CEST0:39
also almost every day some other flights depart after midnight, like just yesterday LO3911 to KRK, departed 00:24 (actually today). Might be, that on the 7th something was particular and WAW really forced closure but there must be evidence for it. So I would come back to the airline asking for this evidence. If they cannot produce said evidence and still refuse to pay, it is easy to sue them because it's just a small claim. In PL, a
postępowanie uproszczone goes for a 100zł fee, no laywer necessary, just filling out the appropriate form and file it at the local court where one lives. If one does not want to take the risk, there are companies filing on one's behalf, but they take a substantial cut from the payment.