iCAL and Thunderbird+Lighting deleting problem

[ggargiulo@ediconsul.it]

HI,
my configuration:
ZCS 5.0.11
Thunderbird 2.0.0.22 + Lightning 0.9
i decide to manage my calendar by Thunderbird creating new remote calendar:
iCal webcal://<servername>/home/<my_username>/Calendar (http[s]://<servername>/home/<my_username>/Calendar is the same).
All functionallities is ok, but if i delete any event Zimbra Web Client not recognize the action, in fact in zimbra log (opt/zimbra/log/mailbox.log) there are follow rows:

2009-06-29 14:43:02,030 INFO  [btpool0-58] [name=<my_username>;mid=27;ip=192.168.2.56;] mailbox - UserServlet (POST): http://<servername>/service/home/<my_username>/Calendar
2009-06-29 14:43:02,030 INFO  [btpool0-58] [name=<my_username>;mid=27;ip=192.168.2.56;] mailbox - UserServlet received file unknown - 8488 request bytes
2009-06-29 14:43:02,109 INFO  [btpool0-58] [name=<my_username>;mid=27;ip=192.168.2.56;] mailbox - Deferred Indexing: successfully indexed 1 items in 19ms (52.63157894736842/sec). (0 items failed to index).  IndexDeferredCount now at 0

i think that the problem is " ... received file unknow ... ".
Can you help me???
Thanks