Oh I'm stupid ...
My problem is solved now.
The problem happened from my NFS server, not from my ESX. My confusion is due to the presence of this strange directory in /vfms/volume who finally disapear when I'm stopped my NFS server.
So I just check my NFS server and ... the User Mapping Service was down ...
Very sorry and thanks for your help. I check "Helpfull Answer" for all of you.
Tom