This bug tends to occur when there is no land enough around a city to set the troops. The system puts them over a city of other player and clicking on the stack can not move them. They get trapped forever.
I show an example in this picture, where blue and yellow stacks are frozen on certain cities and it was impossible to move them anymore.
http://subefotos.com/ver/?8b46b1c83938f667b8d60909e1e54df5o.png