IPSec Fortigate <> Strongswan

Hello Guys, i am facing a challenge that i can only solve with your help. Please support me. I want to establish a VPN connection between my Fortigate 50E and a (Linux) Hosted root server. I've tested it with several instructions but can't get any further. In Fortigate the connection is UP but in the logs it says " negotiate failure progress IPsec phase 2". Strongswan displays the following message: Routed Connections:    FortiGate{1}: ROUTED, TUNNEL, reqid 1    FortiGate{1}: 85.XXX.XX.XXX/32 === 192.168.90.0/24 Security Associations (1 up, 0 connecting):    FortiGate[1]: ESTABLISHED 9 minutes ago, 85.XXX.XX.XXX[85.XXX.XX.XXX]...88.XX.XXX.XX[88.XX.XXX.XX] The hosted root server has no subnet but only a public IP address. The ipsec.conf looks like this: config setup         # strictcrlpolicy=yes         # uniqueids = no conn FortiGate   authby=secret   type=tunnel   auto=route   compress=no   left=85.XXX.XX.XXX   leftsubnet=85.XXX.XX.XXX/32   right=88.XXX.XXX.XX   rightsubnet=192.168.90.0/24   leftfirewall=no   keyexchange=ikev1   ike=aes256-sha256-ecp521   esp=aes256-sha256-ecp521 The /etc/sysctl.conf looks like this: net.ipv4.ip_forward=1 net.ipv4.conf.all.accept_redirects = 0 net.ipv6.conf.all.accept_redirects = 0 If you need more data, just write ;-) Thanks for your help!