I had this happen on check-in for my flight to Bahrain last week. It said it needed to verify my documents even though I had traveled to Hong Kong less than a month prior. When I tried again, it just said I needed an agent to deal with it at the airport. I called UA and she was similarly confused -- some wizardry happened and I was able to check-in within a couple of minutes. She didn't know what she did to allow it and I didn't do anything differently than I had previously, but it worked nonetheless. Very odd.

I just checked-in for a flight to Frankfurt tomorrow and had no issues this time.