Solved! But I think I found a bug in the pfsense software …..... ??
The clue was here:
routing table client:
default 10.138.20.68 UGS 0 40850 1500 sis0
10.138.20.0/24 link#1 U 0 31725 1500 sis0
10.138.20.67 link#1 UHS 0 0 16384 lo0
127.0.0.1 link#5 UH 0 47 16384 lo0
192.168.2.0/24 link#2 U 0 63824 1500 sis1
192.168.2.8 link#2 UHS 0 0 16384 lo0
192.168.4.0/24 192.168.12.2 UGS 0 489 1500 ovpns1
192.168.12.0/24 192.168.12.2 UGS 0 1233 1500 ovpns1
192.168.12.1 link#8 UHS 0 0 16384 lo0
192.168.12.2 link#8 UH 0 0 1500 ovpns1
192.168.18.1 link#9 UH 0 0 1500 ovpnc3
192.168.18.2 link#9 UHS 0 0 16384 lo0
Initially I wanted a tls site to site tunnel and I used this pfsense box as server, I put 192.168.4.0/24 as remote network. Afterwards I deleted it, set up a road warrior network with tls and conigured a shared key tunnel for the site to site connection. 192.168.4.0/24 was removed from the server configuration (at least when I looked at the interface). I wanted to at 192.168.4.0/24 as remote network to the client but it refused to add the route. When I looked at the routing table I noticed that 192.168.4.0 was still connected to the server interface ovpns1! I made a backup of the configuration and there I saw an item <remote_network>192.168.4.0/24</remote_network> in the server config. (Again, in the interface this was nowhere to be seen!).
I removed <remote_network>192.168.4.0/24</remote_network> from the xml and restored the edited config file and…...... it works :).