Didn’t work.
I think it is because older nextcloud packages were obsoleted by nethserver-nextcloud-1.9.0-1.ns7.
I had to remove nethserver-nextcloud and nextcloud packages, then
yum --setopt=obsoletes=0 install nextcloud-17.0.2-1.el7
yum --setopt=obsoletes=0 install nethserver-nextcloud-1.8.4-1.ns7
then update.
Phew. @davidep I really believe that the case should be handled. At minima warning the user that something is going to go wrong.