First make sure you are entering the correct DNS servers for your domain. You can find them by reading this article.

Some local registrars (.de for example) have specific requirements for the DNS servers. Sometimes our server's default DNS is rejected because a requirement is not met.

If this is the case you should post a ticket from your Customer's area in the corresponding DNS category and explain in details which requirement is not met. In most cases we will be able to help you.