This was a BA I.T. issue for a lot of last year and I fell foul of it on several U.S. trips. There was nothing wrong with any of the data I had entered, but the system just took exception to it!
A call to BA was necessary and the BA agent admitted they were well aware of the issue, but they were able to re-enter all the data already in the system and get it accepted. Keep trying via the website, but I've found once it rejects it, the system will keep rejecting it and it needs an agent to override the failing loop I'm afraid.

Pilot37