Finally the problem has been solved. I made the changes you told me, and moreover, to be safe I've set the DNSv6 to the raspberry's IPv6 (even if DHCPv6 was disabled). Now it seems that it filters all requests from each computer, so I can finally reactivate IPv6 on my main Windows too. However, good to know that deactivating the IPv6 does not create problems for the operating system.
Thank you all!