Guests cannot log in

I have a registration and login system on my website, but when they are "guest" they can enter their username and password, they get that everything is correct, but the session does not start.
Only I, having my altervista administration account logged in, can log in, but if I open an incognito window it won't let me log in, I only get that the password is correct and nothing happens.
Before everything worked correctly, but since I updated PHP they can no longer log in. What can I do?

Did you try disabling all the plugins? I tried and everything seems work:
https://i.imgur.com/XpQucofm.png

Bye!