I got the event where I could elevate a maid to a noble lady with great stats, so like any good CK2 player, I murdered my wife to marry the lass with towering... intellect. The plot succeeded against my wife, and I married my lover on the same pause. I believe this is what caused the glitch, as I noticed it occurring shortly thereafter. I also did not receive the royal_aid_duty event, but I don't know if that's part of the bug or if it was changed so that only your first marriage gives that event.
DLC: All but some of the unit packs and 1 or 2 song packs.
Mods: Ruler designer and event/console manipulation to create a starting god character

. As well as a few benign personal tweaks to the defines.lua.

As you can see, it's trying to make the character break up with themselves. I am pretty sure that I was able to break up with my lover earlier in the playthrough, but I may be confusing it with the too many lovers event that makes you choose. Breaking up through this event works fine. Exiting and reentering the game, as well as killing my character to play as someone else, did not remedy it. And the problem persists in subsequent new games.
Also, as to previous post, I am betting the same restrictions that applied to the old lovers events applies to seduction. Hence why we can't seduce close family members. There's probably the "is not close relative" parameter. Just nieces and nephews aunts and cousins I'm afraid.
... XD
A related issue that may not necessarily be a bug, is that AI goes crazy with seduction. My wife was well over 300+(capped at 100 of course) approval of me thanks to attraction modifiers, and did not have the lustful or hedonist trait, but she still cheated on me with every tom dick and harry on the planet. And every AI ruler tried to bone her because of her high stats and good traits. I'm not saying that she should not cheat on me, but I do think the AI of my wife should be a little bit more inclined to refuse with such high approval and non-lascivious traits. But she did have the deceitful trait, so maybe it was just RNG woes.