Yes, I do get a pop-up, but it's very unhelpful, it just says something like "Fatal Error" "the game has generated an error and will..." (the message doesn't end). I can make a screenshot but it won't help you much I think. Shell I?
It doesn't seem to crash in response to any particular action of mine, which fits your explanation (that it happens 5 days after some action). I didn't find a pattern that triggers it, what I know is that I never get very far in the game, with my style of playing it.
Which is this: I focus on money income (which ... ahah, let's say it's subtly hinted to be important), so what I end up doing most often is taking control of cities, planting mines and woodmills as often as I can, recharge exhausted mines and woodmill cards with teas, and upgrade them as often as I can too.
Some comment on the game, for what I've seen: the system looks very cool! The setup is *very* captivating, with this mysterious and ominous focus on money and the titular pact. But also, the game so far strikes me as too easy: everything you do is abundantly rewarded (even just walking), no enemy is ever remotely a threat, significant and permanent upgrades (income, allies, artifacts, spells) seems to be super easy to gain (like when you literally control a city after walking by a few times, or you just collect powerful artifacts on the ground, or your deity packs you with powerful spells in exchange of peanuts in coins). But maybe this is intended, and the challenge will rise later in the game.
EDIT: specific answers: no, it doesn't mention Unreal at all in the Error message, I commented about Unity vs Unreal just because ... the icons of the games give it away. :-D Also, maybe a saving option would not be necessary (barring crashes): I see the nice side of *not* having it. For sure, There Is Only Power worked very well without it. The Daily Challenge mechanism was a dope idea, BTW! Will you replicate that? It makes the well done procedural aspect of the game shine, and it was just brilliant. Oh, BTW, in There Is Only Power it didn't always work as advertised, and often it went back and forth between two scenarios in the same day (I know complete determinism is easier said than done).