if I issue command kolab rename-mailbox and folder has a national character in the name, the error is returned: "mailbox does not exist". The investigation showed that:
pykolab/cli/cmd_rename_mailbox.py file last line should be:

# imap.imap.rename(source_folder, target_folder, partition)
imap.imap.rename(imap_utf7.encode(source_folder), imap_utf7.encode(target_folder), partition)

The original line is commented here.
This happened on Debian Wheezy, pykolab 0.7.15-0~kolab2.