I've been having similar seat selection issues on the Iberia website also. Whenever I click the seat selection button from the manage my booking page a pop-up tells me that there was a server error and to start over from the beginning. I bought my one-way ticket DFW-MAD-BER directly from the Iberia website redeeming Iberia Avios and it's entirely on Iberia metal & operated flights. Pretty frustrating.

Thanks to the previous comments on this thread I went through both the Malaysia Airlines and Royal Jordanian websites and seemed to have success with both of those albeit the seat maps seemed to leave a lot to be desired I was at least able to select seats and wasn't charged in accordance with my OneWorld Sapphire.