I had the same problem on May 5, IAD-LAX.
I did not attempt to check in using curbside/kiosk/counter in advance (previous night), as I had already been upgraded for that flight. I never found out why the problem happened, but in the end, I was able to check in at the counter.

If you'd like further information:
When I arrived at the ticket counter for departure, the agent told me that I could not check in because I owed them $100 change fee. I told them that I had never changed anything on that itinerary. The agent and her supervisor could not identify any change that was made to the ticket except for the upgrade, which should be complimentary in my case (EXP). They kept saying weird, but worked on keyboard to override/waive the fee and gave me the boarding pass.