I have a booking for two people tomorrow. It's booked in First using a GUF2. I can't check-in online. I've called the Gold line who assure me that the booking is ticketed correctly (I have two ticket numbers and the booking hasn't changed since it was booked and the tickets issued). The Gold line suggest I can't check-in because of a "user interface issue". They tried to check me in but got the same message as I do after indicating the number of bags. There is still availability in F (Expertflyer shows F3 A2), so I assume it's not a downgrade, and it's a non-US destination which I understand means it can't be an SSS issues. The Gold line assure me that the seats are reserved so there will be no problem. Any idea what what the issue could be? I've had a look at the main thread but none of the explanations seem to apply.

Thanks in advance.