I have a 2007 423 and had the same problem. Many other owners are experiencing a similar problem. The vertical cushions looked brand new, but any seating surface was cracking and flaking. A dock neighbor who restores high-end automobile interiors checked out the material and rated it very poor from a durability point of view. The backing is quite thin and not stand up to long-term use. I never exposed these cushions to anything that would cause them to deteriorate. Beneteau will not cover these as the warranty had expired and the defect is not structural. Plus, how can you prove you never abused them?
I eventually had the covers replaced with a high-grade automotive vinyl. I was able to salvage most of the foam and backs of the covers and zippers. This held the cost down to approx $3100.
As such, the Duratouch material used by Benetau between 2004 and 2007 may have been of poor quality. If I ever purchase a new boat again, I'll insist on taking the cushions apart and look the material from the inside out.