asterisk users - Nov 2008 - REFER problems with Asterisk and OpenSER

I've set up an architecture in which OpenSER acts as a registrar and
load balancing server for Asterisk machines. I currently have only one
Asterisk machine serving as a Media Gateway.

My problem is that when A calls B, and then A makes a blind transfer
to C, everything works: REFER goes to Asterisk, which processes it,
replies with 202 Accepted, and then it generates a valid INVITE to C.

Hovewer, when A calls B, and then B attempts a blind call to C, things
go awry. REFER from B goes to Asterisk, Asterisk replies with 202
Accepted, and then with 404 Not found in a NOTIFY message.

Only difference between these REFER messages is lack of
Proxy-Authenticate and Proxy-Authorization headers in the on sent by
B. I've read in Asterisk's doxygen doc that all REFER requests have to
be properly authenticated.

Asterisk in 1.4.21.1 from Debian Lenny, OpenSER is 1.3.2.
Any pointers on how to solve this problem? I can give you tcpdump logs
of these transactions if required.

Regards,
Ostrowski Michal