[Dovecot] error 89 Failed to create storage with data

[Nicolas]

Hello,

I have this error message in logs :
dovecot: Nov 07 21:49:34 Error: POP3(manager at mydomain.com): Failed to create 
storage with data: manager at mydomain.com
dovecot: Nov 07 21:49:34 Error: child 27834 (pop3) returned error 89

When I try to retrieve email of manager at mydomain.com kmail says there is no 
new message although this email box is not empty.

I don't understand why. In my dovecot.conf I have 
default_mail_env = maildir:/home/Courrier/%d/%n
with mutt on the server I can read all emails in /home/Courriel and for 
example : /home/Courrier/mydomain/manager/

Does anyone have an idea where I have to look for ?

Thank you for your help and your response

Nicolas