I solved it myself, at least temporarily.
Basically, in my case it was an authentication problem.
The Captcha plugin was not working properly and the user could not be validated on the edit page.
My site does not allow user registration, so I disabled the Captcha plugin and everything works perfectly, without even disabling Sef URLs and CSS and JavaScript compression, as is often suggested.
It's just a Captcha problem!
Now I just need to figure out if this way the present forms are working properly and see how much spam will get to me