Are you trying to access it from LAN or from WAN?
You can try to do a reinstall of nextcloud:
yum reinstall nethserver-nextcloud nextcloud
Then try to access Nextcloud (from Applications panel -> Nextcloud) from LAN.
If still cannot access post the relevant parts (errors/warnings/failures) from /var/log/messages
and /var/log/httpd/ssl_error_log
related to nextcloud.