@sovietishcat
Unfortunately this is nothing you or I can do about. Sounds like when saving the game some information got lost or at least confused. I recommend that you hit “Report to TripleA” once the error occurs and see if there is already a related issue at Github. If not an issue will be created. You could add some information and add the savegame there, then.
Thank you for reporting.
Now for your game, you can try to continue the game from an earlier point of history. I don’t have access to TripleA right now. But when you load your game and go back in game history for example to the end of the turn of the prior power, I seem to recall that you should be able to redo the turn of the current power. There should be something like “…continue from here”. Just try that out. I might give more precise details next week, when I am back to my computers.