This issue has been going on for about a month for me. Units at random get stuck on flat terrain and they cannot move at all, so I have to reclaim/self-destruct them. This seems to be a path finding error. Please find out what's causing it and fix it.
As an example, I'll cite a replay with my friend on Twin Rivers.
At about 8 min, as my Tank Group 2 moves in to flank peter. It goes around the central hill. One of the tanks stops in its tracks on flat ground and it later killed because it cannot retreat.
After the game, peter1984 said that
my commander and units had a mind of there own
When I asked
did your units get stuck on flat ground?
he replied "yes". Then he added:
when i selected my units, they kept double backing on themselves