error while deleting an email from mailbox on virtualmin console

Hi all !!
Recently I discover an error on my public mail server Centos 5.11 when I try to delete an email from a mailbox using Virtualmin GPL 4.13.
I select the domain in the drop-down menu, click Edit Users, open mailbox, check the email, click Cancel and this error appear:
Failed to open temp file /var/spool/mail/"mailbox".del : Permission denied
I use Virtualmin from Lan with root credentials.
I use often this procedure, please, can you help me resolving this?
Thanks a lot.