I'm having the same problem with 2 separate itineraries. I'm traveling from TPA to LAX, and a family member is traveling from ATL to LAX.
I was upgraded at the 5 day window (no NW Worldperks # involved), and she is on an Award Ticket in first class.
Neither of us have been able to check in online since yesterday! I even tried the "change seats" at checkin trick, and it made the "Check in" button activate, but then I got an error saying that I wasn't able to check in online, and to see an agent at the airport.