[Edi]

There is nothing you can do without adding some nuking spells into a mod retroactively and then bombard any and all provinces with large armies with them.

Because you've hit the max unit limit of 140k units in game. It will be impossible to recruit more units, to summon more units or do anything else of the kind until there are less than 140k units on the map. And given the furious way the AI recruits hordes, that's not going to change either.

The only reason you haven't run into the problem before is that you had the chaff disabled from the beginning, but now you're well and truly in it.