[Dovecot] INBOX protected foldername?

[Hajo Locke]

Hello,

have a little problem with folders with name INBOX created by Users.

Some Clients use a Prefix for their Folders. In our old setup (dovecot 1.2) 
it was possible to create a Folder INBOX in mbox-space with some Subfolders. 
These Subfolders of INBOX showed up in Mailclient at toplevel.
Now we updatet to 2.1.7.
In higher dovecot versions INBOX seems to be a kind of protected foldername. 
dovecot is not offering this folder in folderlist. If i rename it, the 
folder is available again.
There are only a few users, so i could fix this manually.
But how to explain this? I did not find a setting etc. which could explain 
this behaviour.

Our Config is pretty standard:
mail_location is unchanged some dovecot versions we use:
mail_location = mbox:~/mail:INBOX=/var/mail/%u

most of conf is out of the box and also used in older versions. only new is 
namespace inbox, but should also be default:

namespace inbox {
  inbox = yes
}

Is there an explaination for this behaviour?

Thanks,
Hajo