I don't know what kind of filament you used, or how big the print is, but my guess is that you sliced with the seam "aligned" and the seam is at that corner. That is where each layer starts and stops, so the extruder spends a little more time there than the other corners, which tends to heat up and soften previous layers of plastic. If the print is small, those previous layers are still pretty warm. Try printing two of those boxes at once and see if it improves.
Since that's the start/stop of each layer, tweaking the retraction settings may be useful. But I'd check the time/temperature issue above, first.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]
