Hello Sebastian,
http://dnsviz.net/d/6v6.de/W9AmtA/dnssec/
Looking at the graph the new KSK (54879) is not signing anything right
now. Shouldn't it sign the DNSKEY records of the ZSKs so that the
chain stays intact when the DS record changed at the parent zone?
You are reading the graph wrong. The new KSK is signing the DNSKEY
RRset. See:
dig @ns1.karotte.org 6v6.de DNSKEY +dnssec
Daniel