I've checked and got the same as you. It appears to be a bug, as a couple of weeks ago I selected seats without difficulty. On the other hand, it is now possible to enter oneworld frequent flyer numbers -- something I couldn't do last time I checked.
Something odd here. I too picked my seat for a flight in August a couple of weeks ago. rj.com seems to allow seat selection but tells me the entire cabin is full; expertflyer.com shows only two occupied seats and all others marked P for Premium only.