Apex domains cannot have CNAME records.
Question and response
Question
dig tds-sharedservices.de. NS
Answer
tds-sharedservices.de. 5 NS ns1.manage-now.de.
tds-sharedservices.de. 5 NS ns2.manage-now.de.
tds-sharedservices.de. 5 NS ns3.manage-now.de.
Authority
The response has no authority records.
Additional
ns1.manage-now.de. 5 A 185.170.25.212
ns2.manage-now.de. 5 A 185.170.25.220
ns3.manage-now.de. 5 A 82.165.68.151
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0