Solved solid layers breaking open

Thanks Mike, it works now.

So here's what I changed:

Layer thickness to 0,3 mm instead of 0,4
Extrusion multiplier 1,15 instead of 1,20
(and I changed the number of solid bottom and top layers to 5 and the perimeter to 2, but they don't have any effect on the problem I had)

I did not have any extra cooling this time, but I'm planning on printing this: http://www.thingiverse.com/thing:431160 and ordering some 40 mm fans. (because I could only find 80 mm in my pc stuff boxes)

So thank you all for helping, I'm not sure which one of those did the trick, but it works now, so that's great!