If you haven’t already you may want to see the dedicated OS5 EX Series subforum where there is much EX2 Ultra discussion. There are several discussions in that subforum discussing specific SONOS issues with OS5.
http://community.wd.com/search?q=Sonos%20category%3A255
Some examples, perhaps there may be a answer in them:
http://community.wd.com/t/after-update-to-os-v5-not-working/258033
http://community.wd.com/t/ultra-ex2-os-5-twonky-server-sonos-issues/260500
http://community.wd.com/t/no-access-to-the-nas-after-upgrading-to-os-5-even-after-resets/258337
