By "1 square", btw, I mean the one single square that the pathfinder chooses.
I have encountered this before and I'm pretty sure thar it's the same thing for 2 stacks etc.
Game:
Outlands, Kauhf, turn 30.
Location:
As far NW one can get without getting into the FOW.
Between two white cities, a 8 DK-stack and a 1 orc-stack.
Try to attack with orc and everything freezes.
Playing with chrome, but have encountered with Opera too.
(When clicking on a closed lake - no ports - with a land unit, it's almost the same thing.)
Edit:

I tried now with a second orc, so there was 3 stacks in a row. Things also got fucked up. Seems like they can't walk through a 8-stack. If it was 7-stack, I believe one unit could come through. 6-stack = 2 units come thorugh.
Moving units back into position!
Go, bug finders, go!