Thanks for all of the quick advice.

It was not really an error message. Is said something to the effect "Unable to complete the check-in process. Please proceed to the airport for check in".

The last time it happened was this past Sunday when trying to check-in for a return flight from SXM (Sint Maarten) and I was still on Anguilla so contacting United was problematic, but it has happened on domestic flights recently and the assistance from the Premier Desk was non-existent. They had no clue. It would have been nice if they transferred me to the Web Site Help Desk.

I've checked my passport info on my profile on the website and it remains correct.

I just spent a bunch of time with the Web Site Help Desk. They were NO help. They cannot or will not look at past reservations. They will only help me if it happens again and I call them. Nice.