Here's the response I got from @deltaassist yesterday when the same thing happened to me:
The only thing I could tell you why is possibly because you were upgraded on the 2nd and 3rd and the tkt wasn't reissued to reflect.
I tried again after the message and it worked fine. Perhaps DL IT has been tinkering with the award calendar again and broke app checkin in the process. Or maybe DL thinks that upgrades are now so rare that it's not really a problem...