Yeah I am at war right now, and I've been transporting a lot of pops around between planets in order to slow down/distract the enemy. (Maybe this is me being punished for being evil, then)

Although I don't think any of the citizens moved from a planet that was then taken by the enemy. None of my cores have been lost. One of the destinations/departures is currently under siege, so maybe that's it?

Did the button also lead you to the Governors screen?

Did you have to give up on the afflicted games whenever this happened to you?

Can you share a save game and we'll take a look?

As for my savegame, yes can give it to you but I'm away from my computer for the next 4 days! So I'll have to give this thread a little bump at that time 

I've encountered the same problem.

I am getting a Lost Citizen, which when selected takes me to my Governors list. As far as I can tell. all my planetary governor positions are filled.

I cannot proceed to the next turn. 

Because things like this can go wrong in turn-based computer games I suggest always setting your save games to every turn.

It is not like 10 or 15 years ago when it took up to15-20 seconds or more to save today there is no discernable lag with computers built in the last 10 years or more.

I even play a RTS that used to take 5 seconds or so to save when it came out 25 years ago that has no lag on a newer computer.