it is not that problem...
I discovered the problem, but I cant fix it, only manually.
The problem is that the .htpasswd file generated:
-rw-r--r-- 1 root root 24 2006-07-20 12:03 .htpasswd
located in the root of each file, when I try to add a new user through ISPConfig web interface it is not updated.
I suspect that ISPConfig cannot touch this file (owner problems? A bug?)
I added manually the username and the password as created before through ISPConfig and now the user CAN access to the stats page.
The problems it is not to view stats, but in the authentication process (and creation of the user).