I would still recommend rerunning memtest. Components wear out, even though your computer may have been fine over the last three years, it could have developed an issue over time. AotS can use large amounts of system memory, which might be using muscles in your PC that hasn't flexed in so long that it forgot that it had them.
After you have confirmed that your system does not have any hardware or driver related issues I recommend saving your games regularly, until you get the crash again. Reload, and see if you can reproduce it. This would help diagnose the issue so that the developers can resolve it if there is an issue in the game.
If your problem persists, you should consider submitting a support ticket.
https://esupport.stardock.com/index.php?/default_import/Knowledgebase/Article/View/530/175/general-ashes-troubleshooting