Update: I just had points for a stay I made in July credit to Southwest, not BMI, despite BMI being my preferred partner.

Long story short: I can't make a reservation online without entering a valid number for some loyalty program. Simply erasing my BMI number still results in the error also reported by others above (I'm going to try to screenshot it next time I get it) - the only way for me to make a reservation online, and thus to get the 500 online booking points, is to enter a number from another airline. I chose Southwest and entered my RR number since I know it off the top of my head. I then went back into my Hilton account to double check that BMI is my preferred partner. It is and was.

I chose BMI as my primary partner the first week of July and haven't ever changed it. So why the Sam Hill are my points posting to Southwest?!

Called Diamond Desk; got a ticket number; I'll report back. If anybody else has had this error, reported it to Hilton, and kept the details of the contact (time of your call to Diamond Desk, which agent you spoke with, or a copy of an email you received from HH on this issue) please PM. Let's try to get this resolved once and for all.