I have had the same problem - booked a reward seat on a HKG-KUL cathay flight for 8 March, it was booked last July through ba.com. Anyway couldn't pick a seat, and tried going on cathay webiste and using finnair mmb with no luck. I couldn't bring up the seat map to select a seat on any website.

This thread reminded me I need to sort it out so I called the cathay centre in the UK this morning. The agent was struggling to find the booking and said there was a system issue - he did take my number and promised to call me back. Anyway, he just has and now sorted. Now have first row window seat in Y.