Update on my little investigation.
Looks like it's not mods in general, but rather a specific MCCC setting, that triggers this issue: changing the game time speed. Well, at least for me.
Computer -> MCCC Settings -> Gameplay -> Game Time Speed (set to 50, default = 25)
However! When the ECO expansion's disabled, the issue is gone, even with the game time speed changed. So I suppose the issue is on the intersection of these two. Each time the notification pops up, when you click OK on it, it opens the Freelance Maker's gig selection panel, with all options greyed out. So it's about that particular branch of the Freelancer job, I guess. (I don't own the DHD expansion, so cannot verify that.)
I'm sending my savefile to EA_Solaire's messages.
Note that it reproduces almost every sim day at around 5 AM, but occasionally it is absent, though it's very rare.
