The block was just a print for an prototype. i printed it very rough and fast. This print was printed turned upside down and full of support material. For this it was okay.
As you can see a few posts before the silver parts came out very nice.
Got no experience with these printers, but might the residual heat in the heavy body be just enough to transfer to those thin walls your are suddenly printing? What if you:
- Printed it on its side
- If support is an issue, make the solid bottom hollow with a few supports in it to carry the flat surface and reduce the density of the base.
- have a cooling fan handy when it gets to that stage
Probably a load of tosh, but the coincidence of the junction of the thin walls with that full layer of hot plastic seems too coincidental to be coincidental. 
Did I just say that?
Yeah they did.
I was wondering about the heat issue as well. I thought increasing the perimeters while lowering nozzle temp at that point. Can be done easily with Octoprint.