We came across the same problem yesterday. It seemed to start
happening after upgrading to Capistrano 2.3.0. Perhaps a regression?
We found some earlier information about this problem. Apparently, one
of the Capistrano tasks runs as user "app" by default, rather than as
the specified user.
We had to add "set :runner, 'username'" to our Capistrano script to
make it work again. I think "set :runner, user" should do the trick,
too.
-Mirko