It is the same problem for me too.
-- This appeared after "For Rent"
-- The Sim I want to merge into the current household is not in the world at the moment.
-- If I move my current SIM to another SIM's household which is not in the world atm, that works.
When the dialog to split/merge households appears, I can switch the sims between the households (and their money). When pressing the checkmark button to confirm, nothing happens, and the button becomes inactive. The dialog is not possible to close, because the close button on the top-right corner is inactive.
The only way to recover from this bug is to either
1) close the game and not save the progress, or
2) move into the not-current-household which is not in the world, and move back to the same house right away (and decorate the house again), and lose the ownership of the rental houses.
I play with Mac and I have the following extras:
-- City Living, Cottage Living, Discover University, Eco Lifestyle, Get Famous, Get to Work, Get Together, Growing Together, High School Years, Horse Ranch, Island Living, Seasons, Snowy Escape, Cats & Dogs, For Rent
-- My Wedding Stories, Dream Home Decorator, Realm of Magic, Jungle Adventure, Parenthood, Vampires, Dine Out, Spa Day, Outdoor Retreat
-- Home Chef Hustle, Tiny Living, Laundry Day, Toddler, Fitness, Vintage Glamour, Kids Room, Movie Hangout, Cool Kitchen, Perfect Patio.
-- Desert Lux Kit, Blooming Rooms Kit, Industrial Loft Kit, Bust the Dust Kit
-- Holiday Celebration