Faced same issue long ago on a test machine. Here’s what worked for me:
If solution does not apply to your case (i.e. no restore point) let me now. I’ve an old VM with the NS7.3 and Nextcloud 12 that I could use to reproduce your issue.
EDIT: reproduced your setup:
[root@server ~]# cat /etc/nethserver-release
NethServer release 7.3.1611 (Final)
[root@server ~]# rpm -q nextcloud nethserver-nextcloud
nextcloud-12.0.2-2.el7.noarch
nethserver-nextcloud-1.1.7-1.ns7.noarch
[root@server ~]# yum update
[root@server ~]# cat /etc/nethserver-release
NethServer release 7.5.1804 (final)

…and solved like this:
[root@server ~]# yum --showduplicates list *nextcloud
Installed Packages
nethserver-nextcloud.noarch 1.2.7-1.ns7 @nethserver-updates
nextcloud.noarch 14.0.3-1.el7 @nethserver-updates
Available Packages
nethserver-nextcloud.noarch 1.2.1-1.ns7 nethserver-base
nethserver-nextcloud.noarch 1.2.2-1.ns7 nethserver-updates
nethserver-nextcloud.noarch 1.2.3-1.ns7 nethserver-updates
nethserver-nextcloud.noarch 1.2.4-1.ns7 nethserver-updates
nethserver-nextcloud.noarch 1.2.5-1.ns7 nethserver-updates
nethserver-nextcloud.noarch 1.2.6-1.ns7 nethserver-updates
nethserver-nextcloud.noarch 1.2.7-1.ns7 nethserver-updates
nextcloud.noarch 10.0.4-2.el7 epel
nextcloud.noarch 13.0.2-1.el7 nethserver-base
nextcloud.noarch 13.0.4-1.ns7 nethserver-updates
nextcloud.noarch 13.0.5-1.el7 nethserver-updates
nextcloud.noarch 13.0.6-1.el7 nethserver-updates
nextcloud.noarch 14.0.0-1.el7 nethserver-updates
nextcloud.noarch 14.0.1-1.el7 nethserver-updates
nextcloud.noarch 14.0.3-1.el7 nethserver-updates
[root@server ~]# yum downgrade nethserver-nextcloud-1.2.4-1.ns7 nextcloud-13.0.6-1.el7
Resolving Dependencies
--> Running transaction check
---> Package nethserver-nextcloud.noarch 0:1.2.4-1.ns7 will be a downgrade
---> Package nethserver-nextcloud.noarch 0:1.2.7-1.ns7 will be erased
---> Package nextcloud.noarch 0:13.0.6-1.el7 will be a downgrade
---> Package nextcloud.noarch 0:14.0.3-1.el7 will be erased
--> Finished Dependency Resolution
Removed:
nethserver-nextcloud.noarch 0:1.2.7-1.ns7 nextcloud.noarch 0:14.0.3-1.el7
Installed:
nethserver-nextcloud.noarch 0:1.2.4-1.ns7 nextcloud.noarch 0:13.0.6-1.el7
[root@server ~]# yum update nethserver-nextcloud nextcloud
Resolving Dependencies
--> Running transaction check
---> Package nethserver-nextcloud.noarch 0:1.2.4-1.ns7 will be updated
---> Package nethserver-nextcloud.noarch 0:1.2.7-1.ns7 will be an update
---> Package nextcloud.noarch 0:13.0.6-1.el7 will be updated
---> Package nextcloud.noarch 0:14.0.3-1.el7 will be an update
--> Finished Dependency Resolution
Updated:
nethserver-nextcloud.noarch 0:1.2.7-1.ns7 nextcloud.noarch 0:14.0.3-1.el7
After that, Nextcloud was working but had to manually update some apps from Nextcloud apps interface, logout, login and re-enable the apps.