I was trying to select seats (or at least see a seatmap) for UA220 tomorrow from SFO to IAD and instead of either the usual seating chart, or the seating chart with no options and a 'seat will be assigned' message at the top, I got the united.com standard error message screen with this on it:

There was an error in processing your request. Please see the message below for instructions.
We are not able to reserve your seats at this time. Please try again later or reserve your seats at check-in.

I tried a few times since last night and have concluded something is up -- I'm not realistically expecting an upgrade (in spite of buying YUA) -- but would like to not end up in 999B. Any idea what this might indicate?