My Schengen visa will be valid on 01 July but I have a flight on 30 June at 22:00. Technically, my visa is not valid when I get on my flight but will be valid when I get to the country. My arrival will be at 00:30. Is that okay or will it be a problem? I'm flying to Poland on WizzAir.
The border guards will only care whether your visa is valid at the time you actually enter the Schengen area. It should not matter how long time you have been traveling by then to get there.
There might be a risk that the airline's check-in/boarding agents at wherever you depart from could be unaware of how the rules work, and therefore try to deny you boarding. This should not happen, but it might be best to ask the airline in advance what their policy in these cases is.