3D Benchy - ABS issues  

3D Benchy - ABS issues

Hi,

I'm new to 3d printing and finished my Prusa I3 MK2S build last week.
I've made various prints in PLA and PETG and they turn out great!

ABS however is giving me a headache, I simply cannot print the 3D Benchy.

Current print settings:
- Prusa black ABS-T
- Standard slic3r prusa ABS settings @ 0.15mm = 255°C@hotend and 100°C@bed with fan disabled

The problem:
The printed ABS looks somewhat smudged/blobbed, the extruder nozzle digs into the printed part after layer X, NOT because of lifting from the printbed (the first layer seems to stick fine), but because of expansion? The printed part remains "soft" during the print (I can see the nozzle warping the lower parts of the print when passing by, over the problem area). As you can see, at a certion layer, the sharp corners start lifting up (see the attachments).

What I've tried so far:
- placing the printer in an enclosure: in my case, a carboard box
- lowering the hotend temperature: 235°C was the lowest setting I tried
- lowering/raising the bed temperature: I tried 85°C all the way up to 110°C
- reducing print speed
- reducing extrusion
- enabling part fan: the print comes out great! But the layer are not bound, the printed part is super brittle...

Since I'm new to 3D printing, I'm running out of options that I can think of... Any suggestions would be great!