Yes thats what happens. Sold out flashes up but then it seemingly lets you select a passenger to book for, but the total remains at £0.00 and will not allow you to proceed.
When we called customer services for them to book for us they said that their portal behaved the same for every date they tried.
Will report back if we have any more success as they will allow us to change our cruise date, just waiting for them to confirm availability.