I just read the Japanese page on the Plextor site, and wrote a quick translation of it. It says the following:
When writing to DVD-R/RW media using a PX-708A/UF with version 1.05 of the firmware, some DVD drives have been found to be unable to playback the discs. Therefore, we have removed v1.05 from the website. (No other functions are affected.)
We deeply apologize to people who have already downloaded this version, and ask that you downgrade back to 1.04. We will make 1.06 available in the very near future, and ask that you upgrade to that, instead.