Hello akkando. there are dozens of reasons for flats. I am not familar with your tire/rim combo but unless you are a Clyde your x25 should be good tires.
there reason for your first flat was likely a "pinch" flat from hitting something on the road (big crack, whole, debris, up or down a curb) with your pressure low. did you patch that tube and reuse it? a pinch flat usually has two small holes close together. if you patched the tube it is possible that you missed one of the holes, there were more that one or two holes or the patch did not work.
or you first flat may have been a 'puncture' the object that caused it is still in the tire. it is a good idea when you are changing a flat to run you fingers around the inside (carefully) of the tire to be sure nothing is still embedded in the casing.
also pinching the tube with a tool can cause a flat.

always pump up your tires before riding (or atleast check if you ride everyday)