asterisk users - Oct 2003 - Trouble with linking 2 Asterisks with IAX

Hi there,

if been sitting on this for over a week now, did a lot of reading but 
still cannot find the solution... it appears that the the remote call is 
mad, but immediately hung up. The scenario:

- a natted SIP client (X-Lite) on the Internet (extension 8602) calls 
server1, dials 98612 and wants to be connected to server2
- the SIP client nat=yes (but NOT canreinvite=no)
- by the way: that SIP client can successfully register with both server1 
and server2 and play with the demo or dial extension 8612
- server2 has a switch statement for server1 and that works fine
- both servers have a static IP
- server1 is to be the primary Internet contact while server2 only has a 
cable connect and is supposed to service an office with natted clients; 
all voice traffic of the office SIP clients shall be run through server2 
to overcome firewall/NAT issues
- both servers register successfully with md5 as friend (via iax.conf and 
correctly shown by "iax2 show peers"; hostname is not specified)
- both servers have the context "default" set, and the extension
dialed -
as well as the demo - is also to be found in the default context

Am I correct that only one of the two servers can "switch" to other
and
that a circular switch setup is not permitted? 
Does the SIP user need to be found in both servers' sip.conf? That would 
be a bit ugly to administrate... :-(
What can be the reason for the "registration refused" error found in
the
IAX2 debug info?

I'll include some more setup and debug info below.