Cannot edit page node

When I try to create a new page's node, it is created with empty fields: in database there is the page's record but the fields are empty.
Other fields, for example "promote to front page" remain unsetted.
If I try to edit an existing page, the changes are not saved.

There're no problems with blog's node: i can create new post, edit olds ecc.

I don't know if the problem is due to this form, but once it's disabled everything work again.