It took me some time to figure this out, so I thought it might be of interest for someone... If you try to set up a site-to-site-VPN between a UTM-1 Edge and a 3rd party appliance, you need to set up TWO tunnels in order for it to work. Looking at the WebUI under Reports>Tunnels, you have to see something like:

Type Source Destination [...]
Phase 1 (UTM-1 Edge) (Remote Site)
Phase 2 [local host] [remote host]
Phase 2 [local net] [remote net]

If the remote site is only a single host (no subnet behind), it would look like this:

Type Source Destination [...]
Phase 1 (UTM-1 Edge) (Remote Site)
Phase 2 [local host] [remote host]
Phase 2 [local net] [remote host]

We successfully connected an Edge X to a Linux Strongswan 4.3.4 using these parameters in /etc/ipsec.conf (remote=linux host, local=edge):

conn remote-to-edge_fw
authby=secret
esp=aes256-sha1-modp1536
ike=aes256-sha1-modp1536
left=[remote host]
right=[local host]
rightsubnet=[local host]/32
pfs=yes
auto=start

conn remote-to-edge_net
authby=secret
esp=aes256-sha1-modp1536
ike=aes256-sha1-modp1536
left=[remote host]
right=[local host]
rightsubnet=[local net]/24
pfs=yes
auto=start

We also managed to connect an Edge N to a Watchguard Firebox (both ends with static IP) in the same way. The order of the two tunnels is important there, see Change Order of Tunnels.