Me too. since last November, or December.
Unfortunately, you couldn't retrieve your domestic site booking record when you register BA FFP number.
YES, 'WHEN REGSITERED BA No.'.
You got the error message when enter your BA FFP number at JAL domestic booking site.
Actually, your FFP number is registered properly.
But somehow, just after FFP number is registered, no longer JAL site couldn't retrieve your booking record.
Some JAL call center people know this problem. but they don't expect near term fix.
So, you have to call JAL call center to change seats, change booking, cancel booking.
If you ask the call center to remove your FFP number, you can access your booking via web.
BTW, I fly more than 40 domestic JAL flights as a BA member annually. It's so annoying.