The mission I am talking about is Rowarty, 'The Sea People'. As soon as I hit 3000 people (or after 9 years ingame), the game stops working.

This is not a compatibility issue - all the other scenario I had played so far worked fine.

Need a fix for this- PRIORITY.

Are you under attack? Rowarty is a common place for game freeze ups caused by invasions, since they're all water based. If it's not that, try just restarting the mission and make sure you are saving regularly.

Hmm, now that I think of it... Could it be something is actually activating this glitch? Like You have to wait for the Sea People invasions to stop before You start building the monuments or else this glitch will happen?

I doubt it, there's few reasons for the game to simply freeze, the most common cause has generally been attributed to sea invasions. I think there was a freeze attributed to something else in the game, might've been monument construction or walker pathing, but can't be sure.

And afaik, the invasions continue throughout the mission, but it's been a looong time since I last played Rowarty so I may be wrong regarding that.

Well, I have found a walkthrough of this scenario on Youtube: https://www.youtube.com/watch?v=Pu1sQPYfbNQ
And although I haven't watched it completely yet, I had noticed invasions did seem to stop after some point. Hence my idea.

And I do know there were games where certain bugs could be activated if the player did something wrong. Hence my question - because after all, if all the previous sea-invasion scenarios worked fine, why did this one suddenly start to crash?