I've successfully shared some mailboxes between users on my server (Debian jessie, dovecot 2.2.13, OX 7.8.3): User A has granted user B to see some folders. When using Thunderbird, the folders are displayed correctly; also on K9-Mail on my Android phone. But unfortunately OX has some strange behaviour:
In The Mail-App I can not see the folders. But, if I go to Settings -> E-Mail -> Button "Subscriptions", I can see the folders. The window contains a folder-structure as follows:
E-Mail (grayed-out)
|- Inbox (grayed out)
   |- Shared (grayed out)
      |- userA@example.com (grayed out)
         |- shared-folder
For testing purposes, I added an additional E-Mail account to OX that connects to the same mail account than the OX default one. In this account, I can see the shared folders both in the Settings and in the normal mail view.

Can somebody tell me, what I am doing wrong?

Here are the relevant parts of my dovecot config:
namespace {
  prefix = INBOX/
  separator = / 
  inbox = yes 

namespace {
  type = shared
  separator = / 
  prefix =  INBOX/Shared/%%u/
  location = maildir:%%h:INDEX=/%h/Shared/%%u:SUBSCRIPTIONS=/%h/Shared/%%u
  subscriptions = no
  list = yes 

Thank you in advance