PROTO_IPSEC_ESP SA not found (maybe expired)

Hi

I've changed from redhat 9 SuperFreeSwan 1.99.8 to suse 9.1 FreeSwan
2.05.
With a few changes done in ipsec.conf it all should have worked fine. 
I can start conns, the freeswan deamon negotiates all its SAs with the
other sides. And I can ping the other sides. But I fail to get any
other communication through any tunnel.
I never have seen before the "PROTO_IPSEC_ESP SA(0x79b5f800) not found
(maybe expired)" error message.

As a matter of fact I recompiled the kernel with ESP, IPCOMP and IPIP
compiled into the kernel not just added as modules. This dropped a
number error messages at the initial Freeswan start-up. However it did
not change the situation. Then I dropped the SuSE FreeSwan daemon and
installed newest StrongSwan 2.1.5 release. No change. So the problem
is probably not deriving from FreeSwan/Strongswan but from a possibly
broken SuSE Kernel 2.6.4-52.
I sniffed a tcp connection to a Windows Terminal Server on the other
end of the tunnel. I've not seen a behaviour like this before:
client		server
tcp:1359, syn --> tcp:3389
tcp:1359, <------ tcp:1024,ack
tcp:1359, rst --> tcp:1024

The tcp sequence and ack numbers are correct in this scenario!
This is quite confusing.

Has anybody got a solution for that?

TIA
Philipp