It depends a bit where you're starting from, but in general Europe is still pretty good in September!
My favorite are Greek spots, in September the mid to low season already starts so the spots get less crowded. My favorite spots in Greece are Limnos (Spot Guide:
http://wakeupstoked.com/kitesurf-limnos-greece/) which is great if you just want to kite, are not looking for partying and just like being at the beach and out in nature, not much around.
Another amazing spot is Prasonisi on Rhodes island in Greece, which has waves and onshore wind on the one side, flatwater and offshore wind on the other. Again, this is no partying spot, so it's all about eat, kite, sleep, repeat there! (I this a video there a few years back:
http://wakeupstoked.com/kitesurf-prason ... ece-video/)
The spots in Egypt are great in September as well! Like El Gouna if you like to party as well and have a little life around it (Spot Guide:
http://wakeupstoked.com/kitesurf-el-gouna/) or Hamata if you just want to kite and dive, not much around but tons of space on the water (Spot Guide:
http://wakeupstoked.com/kitesurf-egypt-hamata/).
There are many more good spots, like Tarifa in Spain (also has a lot of nightlife) or Fuerteventura or other Canary islands.