I am trying to add SSL certificate for my NS gg.cloudns.be

$ host -t NS gg.cloudns.be
gg.cloudns.be name server dane.ns.cloudflare.com.
gg.cloudns.be name server meadow.ns.cloudflare.com.

when try to add certificate, it shows an error banner in the DevilWeb UI.
devil/error/ssl/www/add/le_validation_error

You are showing us that you set NS records pointing to Cloudflare. I don't quite understand what you mean by attempting to generate Let's Encrypt certificates for NS records.