Error 809 when trying to connect Windows 7 with IPsec

Question

Using IPFire 2.17 Core 89 as the IPsec-server, Windows 7 shows the error message "Error 809: The network connection between your computer and the VPN could not be established because the remote server is not responding.". when trying to connect.

From the logs:

charon: 15[NET] sending packet: from <ipsec-ip>[500] to <client-ip>[500] (337 bytes)
charon: 11[JOB] deleting half open IKE_SA after timeout

Solution

The firewall was missing a rule for incoming UDP packets on port 500. This should have been generated automatically but (sometimes) failed.

After adding that rule manually, the problem was solved. The IPfire project also created a patch that will probably fix the root cause.