When an agent is as vague as yours was, this is either a security or a financial verification issue or both and she is not permitted to discuss either with you. Very unlikely to have anything to do with UA's IT if the reservation is in synch (which is 99% of the time the reason for an inability to OLCI).

Taker the agent's advice and give yourself a lot of time for check-in and be prepared for the SSSS treatment at the checkpoint. Also, make sure that you bring the CC used to pay for the ticket.