murder and cross-backend mailbox sharing

[Paolo Cravero]

Hi.
I have a setup with 1 frontend, 1 murder, 2 backends. I am playing with 
cross-backend mailbox sharing, one of the features we value most.

Problem. While I can access shared mailboxes that reside on the same backend 
just fine, with Thunderbird (1.5/linux) I can see shared folders on other 
backends, but when I subscribe nothing is subscribed.

Thunderbird always connects to the frontend machine. All cyrus servers have 
the same Invoca RPMs (v2.3.14-Invoca-RPM-2.3.14-2). Authentication via PAM 
LDAP, if that matters.

I use virtdomains but all accounts are under the same @domain . ACL are 
"anyone lrs".

Summarizing: I can access shared mailboxes on the same backend, but on 
different backends they cannot be subscribed,

Something I should look for?

TIA,
Paolo