I’ve been trying to book a 241 for the last 4 nights and similar problem. I got the ‘multiple tabs’ error. I think this happens when I use the back button to go back a page. The final error message after payment processing and the we’re creating your reservation that I get is ‘Sorry, we have been unable to complete your transaction at this time. We're sorry, we are unable to process your request. Please contact your local British Airways office’