I had that issue a few months ago to book an award flight on SQ (intra Asia). Tried multiple times over several weeks without success. Resorted to calling and similarly, they had no idea why it wasn’t working.
Just be careful when calling up and check your details on the eTicket. Despite using my account points to pay for the flight they managed to stuff the spelling of my name! Not sure how that’s even possibly? They also stuffed the other PAX name.
Fortunately I realised on receipt of the ticket and called back. There would have been a very high chance of being denied boarding…
They had to cancel the entire thing and rebook (including taking a second cash payment and processing a refund on the first payment)….
