Going to assume this was fixed by this commit, as there was no feedback.

Changes have been made in Git (master):

commit d2d6fbbc8886d3e364debe6d431d9d014035cfa8
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Sat Apr 23 02:28:05 2016 -0400

     Fix returning a shared namespace name.

     HIC_Base::listMailboxes returns an array by default with the name in
     the key, and an array as the value. Possibly related to Bug: 14330.

  .../Imap_Client/lib/Horde/Imap/Client/Base.php     |    4 ++--
  1 files changed, 2 insertions(+), 2 deletions(-)

http://github.com/horde/horde/commit/d2d6fbbc8886d3e364debe6d431d9d014035cfa8

I have had difficulty setting the IMAP path prefix in horde webmail 
5.2 on a CentOS 6 server running Courier-IMAP 4.1.2. When creating 
folders I was getting the error, "The mailbox 'New folder' was not 
created. This is what the server said: IMAP error reported by server."

The imp_imap debug log shows that it is trying to create folders
without the `INBOX.` prefix, which I think is the reason it is failing:

        S: * OK [CAPABILITY IMAP4rev1] Courier-IMAP ready. Copyright 1998-2005
        Double Precision, Inc.  See COPYING for distribution information.
        C: 1 LOGIN quinn@example.com [PASSWORD]
        S: 1 OK LOGIN Ok.
        C: 2 LSUB "" New folder
        S: 2 OK LSUB completed
        C: 3 LIST "" New folder
        S: 3 OK LIST completed
        C: 4 CREATE New folder
        S: 4 NO Invalid mailbox name.
        C: 5 LOGOUT
        S: * BYE Courier-IMAP server shutting down
        S: 5 OK LOGOUT completed

I *am* able to create a folder if I manually include the IMAP prefix
to its name, i.e. I go to Folder Actions > Create Mailbox and then use
a folder name literally as `INBOX.New folder`. The new folder is created
with that name literally and `INBOX.New folder` is created on the server and
is displayed in IMP's sidebar. =\

As far as I can tell, this is configured using the 'namespace' option
in backends.local.php, but I have tried the following variations with
no success:

      'namespace' => array('INBOX.')
      'namespace' => array('#INBOX.')
      'namespace' => array('INBOX/')
      'namespace' => array('#INBOX/')

I believe the correct method of setting the IMAP path prefix is to 
define the following in backends.local.php:

     'namespace' => array('INBOX.'),

However, that alone has not solved the problem. I may have found 
issues with horde/imp or our server configuration. Now, even with the 
namespace specified thusly, IMP is still sending IMAP commands without 
the namespace (i.e., `CREATE New folder` instead of `CREATE INBOX.New 
folder`).

I have read the source code of horde/imp and found the following 
changes were required for this to work:

## /var/www/html/horde/imp/lib/Imap.php : 474

The `getNamespace()` function defined here would return null when the 
specified mailbox was the default, despite `$this->getNamespaces()` 
returning a valid result. To solve this I added the following at line 
474:

     if ($mailbox === self::NS_DEFAULT && $ns->count() == 1) {
         return end(end($ns));
     }

## /usr/share/pear/Horde/Imap/Client/Base.php : 742

Here, the name of the namespace is an array and would result in an 
array-to-string conversion, with the resulting namespace name `Array` 
instead of the correct name `INBOX`. I've added a tertiary clause to 
test if it is an array before assigning the value. Secondly, the 
namespace name lacking the delimiter would result in the use of folder 
names such as `INBOXNew folder` instead of `INBOX.New folder` (because 
in other parts of the code where the namespace is used the namespace 
is concatenated with the folder name with no delimiter), so I added 
the delimiter to the name here. The following is a modification of 
line 742:

     $ob->name = (is_array($val) ? end($val) : $val) . $val['delimiter'];

With these two changes, horde/imp is now successfully creating 
folders, displaying folders, auto-creating special folders, and moving 
messages between folders. The IMAP transactions now send the prefix 
correctly:

        S: * OK [CAPABILITY IMAP4rev1] Courier-IMAP ready. Copyright 
1998-2005 Double Precision, Inc.  See COPYING for distribution 
information.
        C: 1 LOGIN q@asdf.com [PASSWORD]
        S: 1 OK LOGIN Ok.
        C: 2 LIST "" INBOX.
        S: * LIST (\Marked \HasChildren) "." "INBOX"
        S: 2 OK LIST completed
        C: 3 LSUB "" "INBOX.New folder"
        S: 3 OK LSUB completed
        C: 4 LIST "" "INBOX.New folder"
        S: 4 OK LIST completed
        C: 5 CREATE "INBOX.New folder"
        S: 5 OK "INBOX.New folder" created.
        C: 6 LSUB "" "INBOX.New folder"
        S: 6 OK LSUB completed
        C: 7 LIST "" "INBOX.New folder"
        S: * LIST (\HasNoChildren) "." "INBOX.New folder"
        S: 7 OK LIST completed
        C: 8 LOGOUT
        S: * BYE Courier-IMAP server shutting down
        S: 8 OK LOGOUT completed

If I have discovered bugs in horde/imp 5.2, please let me know how 
best I can contribute these changes (pull request on github?) or if 
you believe this is a problem with my Courier IMAP configuration, I 
would appreciate some help identifying if that may be the case.

Copied below is IMP's imap log output during a successful folder 
creation operation (also so you can see imap capabilities, etc).

------------------------------
S: * OK [CAPABILITY IMAP4rev1] Courier-IMAP ready. Copyright 1998-2005 
Double Precision, Inc.  See COPYING for distribution information.
C: 1 LOGIN q@asdf.com [PASSWORD]
S: 1 OK LOGIN Ok.
C: 2 LIST "" INBOX.
S: * LIST (\Marked \HasChildren) "." "INBOX"
S: 2 OK LIST completed
C: 3 LSUB "" "INBOX.New folder"
S: 3 OK LSUB completed
C: 4 LIST "" "INBOX.New folder"
S: 4 OK LIST completed
C: 5 CREATE "INBOX.New folder"
S: 5 OK "INBOX.New folder" created.
C: 6 LSUB "" "INBOX.New folder"
S: 6 OK LSUB completed
C: 7 LIST "" "INBOX.New folder"
S: * LIST (\HasNoChildren) "." "INBOX.New folder"
S: 7 OK LIST completed
C: 8 LOGOUT
S: * BYE Courier-IMAP server shutting down
S: 8 OK LOGOUT completed