I moved him to the side wall in the entrance chamber of the base. I think the game allowed me to move him one tile too many, that is into the wall. After that I couldn't move him at all. When I selected it and clicked anywhere no movement path appeared.
I lost the assault
Edit: Slightly unrelated, but I replayed that mission and this time the hunter was blown up right at the entry corridor and It was impossible to bypass it forcing me to abandon the mission. A bit of a level design bug there.