I agree with piercet’s diagnostic of the layer seperation. Increasing the extrusion temps by a few degrees will keep the plastic liquid/molten longer after leaving the hotend… which should help with adhesion to the adjacent layer.
In addition to increasing the temps, I also like to keep cooling from the fan fairly low 30-50%. Cooling ABS too fast can lead to contractions. Bridging is the only time I like to blip the fan to 100%… locking the filament in place.
The other thing to consider is the airflow from the fan, when it hits the edge of the project perimeter some of the air will deflect downwards along the wall. Thus further cooling the lower part of the project…
I don’t have much insight into problems with the lower side of the box. It could be one side lifting and causing the project to press against the nozzle. The PEI sticks to ABS pretty well and is very forgiving. Give the PEI bed a scrub with fine grit sandpaper (I’ve been using a Mr. Clean Magic Eraser which has very low grit, but removes oils). Do you remember if that side of the box was on the left / right side of the bed or front / back? That can help you determine extruder is uneven along X or Y axis. To truly diagnose, print a few 10x10mm or 20x20mm calibration cubes… or whatever length to help reproduce the issue.
Anyway, there’s a lot to consider, but you’ll find a sweet spot! Keep at it.