Apex domains cannot have CNAME records.
Question and response
Question
dig ess.de.barracuda.com. NS
Answer
ess.de.barracuda.com. 4 NS ns-651.awsdns-17.net.
ess.de.barracuda.com. 4 NS ns-1430.awsdns-50.org.
ess.de.barracuda.com. 4 NS ns-2001.awsdns-58.co.uk.
ess.de.barracuda.com. 4 NS ns-473.awsdns-59.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0