It's not just you.

I was trying to book JNB-WDH-CPT (which on BA is JNB-WDH-(JNB)-CPT) - essentially two one-ways - and got the same message. That was after I'd tried the 'private window' function as ba.com wasn't behaving properly. I could choose the flights and then got the Error message.

Again using 'private window', BA's South African site let me progress.

Curious.