It seems roundcubemail from nethserver-updates wants to update to the obsoleted roundcubemail from the nethserver-roundcubemail-next package from nethforge.
Workaround:
If you want to keep Roundcube 1.4 LTS then you may disable the NethForge repo in /etc/yum.repos.d/NethForge.repo by setting enabled to 0 until it’s fixed:
[nethforge]
name=NethForge $releasever
mirrorlist=http://mirrorlist.nethserver.org/?release=$releasever&repo=nethforge&arch=$basearch&nsrelease=$nsrelease
#baseurl=http://mirror.nethserver.org/nethserver/$releasever/nethforge/$basearch/
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-NethForge-7
gpgcheck=1
repo_gpgcheck=1
enabled=0 # <-- set enabled to 0
enablegroups=1
EDIT:
Instead of disabling the NethForge repo you may add following line to exclude the update for now:
exclude=nethserver-roundcubemail-next
If you want to upgrade to Roundcube 1.5.2: (Warning: I don’t know if there’s a simple downgrade in case of failure)
rpm -e --nodeps nethserver-roundcubemail roundcubemail
yum install nethserver-roundcubemail-next