Along with all the other guild-related bugs introduced in 10.2.7, I submit the following:
Shroud of Cooperation is not available for purchase from guild vendor (Perry Charlton in Boralus, haven’t checked other vendors yet as I suspect it will be the same for them, too)
The guild has the necessary achievement for the cloak to be on sale (all of my other alts in the guild already have it)
The character in question is a long-time Exalted member of the guild and so meets any purchasing requirements (requires Honoured with guild)
…
…
EDIT:
I forgot to mention that I think THIS ONE HAS BEEN FIXED now (it’s working for me, at least)
I suspect that it was fixed along with the Guild Bank and Guild Roster problems around May 15th.
…
…
Other Guild Achievement-related problems have NOT yet been fixed, however.
See here, 10.2.7 Guild Achievement Broken
This post by Faethor also has a good list of achievements that were broken at the same time