I assume you connect directly from your NS to the ubiquiti router, both have public IPs, no port forwarding.
You can get the IPSEC log on Nethserver with:
journalctl -u ipsec.service
In general you need same settings(encryption, compression, PFS etc.) on both sides.
I did some research and found that it may be a wrong PSK or a firmware issue:
Nethserver IPSEC Docs:
http://docs.nethserver.org/en/v7/vpn.html#ipsec
http://docs.nethserver.org/projects/nethserver-devel/en/v7/nethserver-ipsec-tunnels.html