As commented before, disabling rpm-deltas made the update work:
yum --setopt=deltarpm=0 update nethserver-cockpit
I don’t know if it is the correct way to test this or it’s of any use for the repo maintainers, but when applying deltarpm to nethserver-cockpit or when generating delta-rpm from the packages the resulting checksum differs:
# applydeltarpm -r nethserver-cockpit-0.1.1-1.ns7.noarch.rpm nethserver-cockpit-0.1.1-1.ns7_0.2.0-1.ns7.noarch.drpm nethserver-cockpit.rpm
# makedeltarpm nethserver-cockpit-0.1.1-1.ns7.noarch.rpm nethserver-cockpit-0.2.0-1.ns7.noarch.rpm nethserver-cockpit.drpm
# sha256sum *.drpm
0ff5da42a7474b544008cfbc22694a17700a2a8e2b707aa96fcdc5698a9537e4 nethserver-cockpit-0.1.1-1.ns7_0.2.0-1.ns7.noarch.drpm
73b91db4a5b0fe0b9b112d2180035b4c57ec8bf2a1ca916a2da3740a4089892e nethserver-cockpit.drpm
# sha256sum *.rpm
a9bd1125bc989dc382cda9ad607d6c460421e3a55fdac1c4f4bc842deabaa22c nethserver-cockpit-0.1.1-1.ns7.noarch.rpm
fd4e709cab94499fd046f4b2ad82b6ff14a2a5cbef36c734288950a439b06c67 nethserver-cockpit-0.2.0-1.ns7.noarch.rpm
61e8373a2536e6cdb87ed87c1400d6e90a7ed3c0eef19e5943ffc98df961600a nethserver-cockpit.rpm # shouldn't it be the same as nethserver-cockpit-0.2.0-1.ns7.noarch.rpm ?