I’m also having the same issue checking in for a flight to MIA tomorrow. For both of us. I was in MIA last month, ESTA all fine. BA was also really struggling with me adding API data and FF number. So assuming it’s all just a bit wonky on their end today. And hopefully all ok tomorrow.

I don’t know if it’s because we changed this outgoing flight - all still under the same booking but not sure if that has done something. I never have these issues.