Every time I begin a turn I get 3-7 messages (it is usually however many times my planets are attacked in a turn) saying "Cannot Invade" and that the planet is empty and needs a transport to invade it.  Even though I don't even have any military ships, and haven't attacked anything at all the entire game.  I think it is related to the "Drones" custom civilization perk (where you get automatically regenerating drones on all of your planets and starbases-also this perk is under powered compared to most others-it should at least have attack and defense of each type) since it seems to occur when I have a planet that has no ships on it but it gets attacked anyway because it has drones on it.  And of course the drones lose every turn because they're pathetically weak.  Also, these messages tend to pop up at the same time as "under attack" messages, which seems to confuse the game (long lag, freezes and sometimes crashes).