I made the CoreSwitch on the Client-site be the DHCP Relay and everything workes flawless. This is acceptable for me, however, since I am going to rely on pfSense for a big data center / outsourcing project with several sites attached in the future, I would appreciate if someone can point me to what happened here. I am otherwise very happy with this product but this leaves a bad feeling without knowing what was going on. The articel (http://doc.pfsense.org/index.php/Why_can't_I_query_SNMP,_use_syslog,_NTP,_or_other_services_initiated_by_the_firewall_itself_over_IPsec_VPN%3F) explaines to me why a static route was needed to get the packets into the tunnel (even though I would be very appreciative for more details on this "issue", which seems to be by design), but I don't really get why the packets go into the tunnel and disappear on the way back.
Also, is there a way to debug traffic inside the ipsec-tunnel? Probably not using a packet sniffer since the traffic is encrypted, but is there some other, possible ipsec related, logfile etc. to tell me when and why packets get dropped and for what reason?