coincidentally, a few weeks ago, on my outbound, I too had this issue. It was AA to BA, but it already was NOK on the AA first flight. When checking in online, it kept asking me for my birth date, which I correctly filled in, but the system still did not manage to check me in.
At the airport, the agent had to try a few times to update that - as it also affected my KTN showing up. This was my first time facing this issue....at least it never lead to any major issues.

Cheers!