I ran into this issue twice in the same game.

There first time was during the start of the game I had my scouts looking around the map. I wondered into an AI player's output that was not connected to a city yet. Then the next turn they connected it to a city and I was trespassing, but could not move, unless I wanted to declare war. My units were forced to sit their and slowly die. 

The second time this happen to me was after making peace with another player I still had units in their territory and the moment I made peace I was trespassing and could not move my units. In this case I did not have the option to declare war to move my units as I had just made peace. My unit could just not move. It is rather frustrating to loss experienced troops that way.

I wish you all the best of luck in figuring out this bug.

Best,

Zobot

Temporary Files.zip

Player.log   

Have you tried to set a path by clicking outside of the opponent's territories?

You're welcome!

@Daarkarrow Still waiting for the »Help« sub-section...