Anybody familiar with the airspace above that area?
Very. Right there it's class G to 1500' (which was roughly the top of the fog layer), then class E (Rotterdam TMA) to FL55, then class A. So right there there would not have been an airspace issue if they would have climbed.
However, just a few miles North (the direction they were heading) the Schiphol TMA (class A) starts at 1500', and the Hotel arrival into Rotterdam starts there, also at 1500', so it's uncommon to be much above 1500' over there, considering their direction of travel and destination.
I was aerobatting in the Rotterdam TMA, and heard Rotterdam approach trying to contact the PH-SKJ. However there was no indication whatsoever that there might've been something wrong. Just a routine 'are you on this frequency?' call.