I have never......ever.....EVER.....had ANY luck (we're talking absolute zero here) with the multi-stop tool in a reward booking.
I mean, let me make it even more clear: I can do a roundtrip search, find the cities it wants me to stop in, then put the EXACT SAME ITINERARIES that THEY came up with into the engine, and have it come back and say, "Your search criteria violates (some) rule."
Very annoying, but then there are LOTS of idiosyncrasies with co.com award booking that are very annoying..... like all of my purchases bringing up the error message, then forcing me to make the booking over the phone, AND pay the phone booking fee.

But yes, your problem seem pretty typical to me.