But when i try to create cert or renewal i get connect error for www.domain.com
certbot certonly --webroot --webroot-path=/var/www/domain.com/htdocs -d domain.com -d www.domain.com
Remove the AAAA record for www.starenio.gr from your DNS server and try again. The other option is to configure the ipv6 address for your domain and the web server correctly to serve your domain using ipv6 too.
Note: 2a01:4f8:c0c:fa1:: is not a valid ipv6 address, is the subnet 2a01:4f8:c0c:fa1::/64 assigned to you by Hetzner.
Let's Encrypt try to reach your DNS server every time to check what your records are but doesn't cache them, well it does some cache but as far as I know, only 30 or 60 minutes. Indeed if you remove the AAAA record, you can wait 5 minutes and try again.
I supposed that if you used that subnet is because it is the one assigned to you. If you have a dedicated server yes, you should have a /64 subnet assigned to you at no cost (you can check it from the robot web page -> Servers -> Select your server -> IPs tab and you will see the assigned subnet).