Ok, it's amazing.
I found that if i write, for example, "app/etc/local.xml" within an article, wordpress gives me a 403 error page during save or preview operations.
Now, I tried to decompose the string and i found that the problem is that " /etc/ " piece!! No errors for " etc ", " etc/ ", just for " /etc/ "!
I tried to activate debugging options also but no informations comes back about this error.
I tried to deactivate plugins and so on, but nothing happens!!!
The apache error_log says:
ModSecurity: Access denied with code 403 (phase 2). Pattern match....
It seems that for some kind of string within my posts, mod_security apache's module try to stop an injection.