While dig shows the correct nameservers
% dig caas-reinigung.de NS
caas-reinigung.de. 86400 IN NS dns1.p01.nsone.net.
caas-reinigung.de. 86400 IN NS dns4.p01.nsone.net.
caas-reinigung.de. 86400 IN NS dns3.p01.nsone.net.
caas-reinigung.de. 86400 IN NS dns2.p01.nsone.net.
whois does not
% whois caas-reinigung.de | grep 'Nserver'
Nserver: ns1030.ui-dns.org
Nserver: ns1062.ui-dns.com
Nserver: ns1066.ui-dns.de
Nserver: ns1086.ui-dns.biz
Suggest reading through the following support guide
as well a checking out other threads in these forums related to IONOS and also the IONOS Help Center