Are you sure this is an OctoPrint issue?
I don't know what the problem is, so let's ask....
does the printer freeze when it prints the corners?
not quite sure what you mean by that?
the burn marks of the print on the image are in two of the corners, aren't they?
yes.two of the corners
Most of the time when burn marks occur it's because the printer paused for some time at those locations.
Do you know if that was the case for this print?
Another option would be that this is just black filament that was stuck in the nozzle. That happens sometimes when you use for example petg filament that needs a high print temperature and then print with pla on a very low temperature.
But I doubt that's what happened here - it's too regular and not random as you would expect it from stuck filament.
A third thing that comes to mind is that the overhang was too much, resulting in "loose" corners. If the nozzle was "dirty" (old or burnt filament residue), it could have been wiped on these "loose" corners.