rename not possible

[Kerstin Espey]

I want to rename several mailboxes from mixed-case to lower-case, for example:
rename user/TEST user/test
but always get an error:

localhost> sam user/TEST cyrus all
localhost> rename user/TEST user/test
renamemailbox: Operation is not supported on mailbox

Renaming subfolders is no problem.

I have cyrus 2.1.19 from the debian-package.

Does anyone know what is going wrong? 
-- 
Thank you for your help.
Kerstin