Hi Jana,
In the end this what I did. I sliced my numpy array by extracting only those values that were falling in the [0, -30] interval (as I have checked the values of my pixels in SNAP and most of them were under 0). I have managed to plot the histogram. I guess the problem was the size of the data, but I still don’t understand if this was for sure. My initial array had 161044520 elements and the new one is 121587004 elements. There is a considerable difference of almost 40 mil elements, but I given the sizes, I don’t see why this was such a problem.
Anyways, thank you! 
I am still beginner level and learning a lot, so any advice is much appreciated.
Cheers,
Cristina