ssllabs.com IPv6 issue: Unable to connect to the server (nginx)

This error drives me crazy: Running nginx on Debian Buster. ipv4 works fine, but testing with ipv6 throws:

Unable to connect to the server

AAAA Record is available and valid. Connection with http://ipv6-test.com and with Powershell (Test-NetConnection) from a DualStack Host was not successful.

Version check with nginx -V:

nginx version: nginx/1.14.2
built with OpenSSL 1.1.1d  10 Sep 2019
TLS SNI support enabled
configure arguments: --with-cc-opt='-g -O2 -fdebug-prefix-map=/build/nginx-Cjs4TR/nginx-1.14.2=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -Wdate-time -D_FORTIFY_SOURCE=2' --with-ld-opt='-Wl,-z,relro -Wl,-z,now -fPIC' --prefix=/usr/share/nginx --conf-path=/etc/nginx/nginx.conf --http-log-path=/var/log/nginx/access.log --error-log-path=/var/log/nginx/error.log --lock-path=/var/lock/nginx.lock --pid-path=/run/nginx.pid --modules-path=/usr/lib/nginx/modules --http-client-body-temp-path=/var/lib/nginx/body --http-fastcgi-temp-path=/var/lib/nginx/fastcgi --http-proxy-temp-path=/var/lib/nginx/proxy --http-scgi-temp-path=/var/lib/nginx/scgi --http-uwsgi-temp-path=/var/lib/nginx/uwsgi --with-debug --with-pcre-jit --with-http_ssl_module --with-http_stub_status_module --with-http_realip_module --with-http_auth_request_module --with-http_v2_module --with-http_dav_module --with-http_slice_module --with-threads --with-http_addition_module --with-http_geoip_module=dynamic --with-http_gunzip_module --with-http_gzip_static_module --with-http_image_filter_module=dynamic --with-http_sub_module --with-http_xslt_module=dynamic --with-stream=dynamic --with-stream_ssl_module --with-stream_ssl_preread_module --with-mail=dynamic --with-mail_ssl_module --add-dynamic-module=/build/nginx-Cjs4TR/nginx-1.14.2/debian/modules/http-auth-pam --add-dynamic-module=/build/nginx-Cjs4TR/nginx-1.14.2/debian/modules/http-dav-ext --add-dynamic-module=/build/nginx-Cjs4TR/nginx-1.14.2/debian/modules/http-echo --add-dynamic-module=/build/nginx-Cjs4TR/nginx-1.14.2/debian/modules/http-upstream-fair --add-dynamic-module=/build/nginx-Cjs4TR/nginx-1.14.2/debian/modules/http-subs-filter

/etc/nginx/sites-available/default looks like

server {
    listen 80 default_server;
    listen [::]:80 default_server;
}

netstat -tulpn says

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      656/nginx: master p
tcp        0      0 0.0.0.0:443             0.0.0.0:*               LISTEN      656/nginx: master p
tcp6       0      0 :::80                   :::*                    LISTEN      656/nginx: master p
tcp6       0      0 :::443                  :::*                    LISTEN      656/nginx: master p

Firewall is open ufw status:

80/tcp (v6)                ALLOW IN    Anywhere (v6)
443/tcp (v6)               ALLOW IN    Anywhere (v6)

Assistance very appreachiated!