Hmm. The only two things that are different is my mount target is in under /tmp while yours is under /opt, which shouldn't matter, and I use winnt (which I pulled out of sony's .so files) rather than 'mixed'. I'll try your settings and see if that helps. Still looks like a bug in SONY's driver or the VFAT filesystem driver for that kernel version.
Thanks for your input kartu! Knowing it used to work on the 505 is useful knowledge