One of the domain's nameservers says cpanel.overalwifi.nu doesn't exist.
http://dnsviz.net/d/cpanel.overalwifi.nu/WxfTXA/dnssec/
For the mail and www subdomains, some of the nameservers return different IP addresses, or don't work, but none of them appear to return NXDOMAIN.
http://dnsviz.net/d/overalwifi.nu/WxfUKw/dnssec/
http://dnsviz.net/d/mail.overalwifi.nu/WxfULQ/dnssec/
http://dnsviz.net/d/www.overalwifi.nu/WxfULw/dnssec/
You may need to sort out the DNS issues before the validation will work. Or at least work reliably.