Another information is that the internal IP used by the OpenVPN tunnel and therefore the source address of all traffic that comes from the tunnel is another IP range (Lets call ist VLAN*4, regardless that it is not a Vlan in the sense of 802.1Q).
Therefore The Source address of the Package reaching the host in VLAN1 is coming from something outside its Network and is routed back to its default gateway which is the SRX.
Therefore the Traffic should be as follows:
VPN client (VLAN*4) -> PFSENSE (VLAN1) -> SRX (VLAN1) -> Server (VLAN1) -> SRX (VLAN1) -> PFSENSE (VLAN1) -> VPN Client
While I type these words I see that in mtr I only see
(VPN Client, not shown) -> SRX (VLAN1) -> Server (VLAN1). I Wonder why the Pfsense does not show as router.