Greetings all. @WarningU2 and I play cooperative MP on a weekly basis and have been doing so for a number of years now. Through trial and error, we figured out how to best (or at least better) address Out-of-Sync issues during play that cropped up over its last couple of updates: have the host exit the game and clear their Download Cache through Steam's settings. This should be of particular focus where all players desync, as it strongly suggests an issue with the host; the sooner this is done in play afterward the first such desync, the sooner they tend to resolve and are likely to stay resolved. In Steam, go to View>Settings>Downloads>CLEAR DOWNLOAD CACHE button. Afterwards, you will need to log back onto the service; then, when going to launch the game again, expect a) a pop-up of a dialog box and b) the need to press the PLAY GAME button to do so, following a few seconds-or-so delay. These steps can be repeated if, and as need be, as play progresses thereafter; while there is certainly no harm in non-host players doing this too, we have observed it only being imperative that the host do so. Relatedly, to continue play we recommend the host rehost the game from a recent manual, or even the most recent auto, save file. Hope this helps with any desyncs you encounter in your own MP sessions. Keep on Civ'in!