Invasion causes AI turn does not end

Played about 41 hours until some humans invaded one of my undefended planets. The invader turn never ends. The planet does not change allegiance. I cannot save or even load, barely exit the game (to desktop only), and can navigate menus. it was the first time I was ever involved in an invasion. This is extremely frustrating. I'd like to finish this game. I don't dare to start a new one by fear I would get this bug again, although I can't find anyone having complaining of such a bug in years.