I've had this bug since .952 and it might be known, but I haven't seen it included in any changelogs until now so here it is.
I think it happens when an army has a destination but it encounters something on the way (monster, quest, goodie hut etc.) and then it gets stuck running in place in the tile where the action happened, with the old destination still set, but unable to move. The solution is to toggle Explore twice, then the unit stops running in place and I am able to move it as I wish.

Providing a savegame is useless, since upon restart, the issue disappears.