only write at 2.4x as the log you posted was from an 8x burn can only take your word for it that you tried lower speed, as you say you have the latest firmware, there is not much more you can do.
You may have a batch of verbs that are not good, slim odds on that but suppose it could happen.
See shop(company) you bought it from to see if you can swap it for a pioneer 112, tell them it's failing to write to very expensive verbatim dual layer media.
Have you got something else running like anydvd or dvd43 or any other util that accesses drive ?