Thanks for the reply! I was just worried I wouldn't be able to complete other quests but it seems like I still can. FWIW I got the first "something odd" notice right after I signed the second treaty. And I've been lurking on these boards for many years, but I've never before done something Randomizer says is impossible.
Yes I also had the calamity return after only 2 vassals, on day 104. I have "reclaiming the Ahriel" and "The calamity returns" as quests. (As well as various other Ahriel quests.) Is this a bug? Everyone says you have to complete all 3 "reclaiming the <country>" quests first but I didn't.