Population loyalty
 

Although I realized that SE V has population loyalty besides happiness, I never payed too much attention to this...
until...
I got two colonies rebellions at the same turn!
As this resulted in two new empires, I got the "turn processing" game hanging next turn. I believe that this might be the most common cause of the reported game hanging by other players.

I then looked at loyalty a little bit closer:
- There seems to be no direct possibility to increase the population loyalty as you can do this with troops for the happiness.
- The only modifiers I found were planet conditions, population happiness, government type, race and civic achievements.
- There is no warning if the loyalty of a colony drops to 0% and in the colony list you can't display the loyalty for an overwiew.
- You can have jubiliant population with 0% loyalty as this was the case in my game.