How do I get push notifications to work correctly?

Using Snow Leopard Server 10.6.8. I turned on the iChat and push notification servers, added myrealdomainname.com and mycanonicaldomainname.com and pubsub.mycanonicaldomainname.com to the iChat service host domains, and then turned on push notifications in the iCal service. But whenever I try making a change on the server using a third-party CalDAV client, iCal is running but is not notified of the change.

The iChat server log says the service is logging in just fine:

May 23 15:04:18 69 jabberd/c2s[62947]: [8] SASL authentication succeeded: mechanism=DIGEST-MD5; authzid=com.apple.notificationuser@mycanonicaldomainname.com, TLS negotiated

May 23 15:04:18 69 jabberd/c2s[62947]: od_auth_check_service_membership: checking user "com.apple.notificationuser" access for service "chat"

May 23 15:04:18 69 jabberd/c2s[62947]: od_auth_check_service_membership: mbr_check_service_membership returned 2

May 23 15:04:18 69 jabberd/c2s[62947]: od_auth_check_service_membership: no access restrictions found

May 23 15:04:18 69 jabberd/c2s[62947]: [8] bound: jid=com.apple.notificationuser@mycanonicaldomainname.com/icalserver.72bd94580b794782ac0eb9477c5dcef9

May 23 15:04:18 69 jabberd/sm[62943]: session started: jid=com.apple.notificationuser@mycanonicaldomainname.com/icalserver.72bd94580b794782ac0eb9477c5dcef9

So, I set iCal's log level to "debug" and saw this error:

2012-05-23 15:04:18-0700 [-] [notifications] 2012-05-23 15:04:18-0700 [XmlStream,client] [twistedcaldav.notify.XMPPNotificationFactory#debug] SEND: <presence/>

2012-05-23 15:04:18-0700 [-] [notifications] 2012-05-23 15:04:18-0700 [XmlStream,client] [twistedcaldav.notify.XMPPNotificationFactory#debug] SEND: <iq type='get' id='H_7'><query xmlns='jabber:iq:roster'/></iq>

2012-05-23 15:04:18-0700 [-] [notifications] 2012-05-23 15:04:18-0700 [XmlStream,client] [twistedcaldav.notify.XMPPNotificationFactory#debug] SEND: <iq to='pubsub.mycanonicaldomainname.com' type='set' id='H_8'><pubsub xmlns='http://jabber.org/protocol/pubsub'><publish node='/Public/CalDAV/myrealdomainname.com/8443/'><item><plistfrag xmlns='plist-apple'/></item></publish></pubsub></iq>

2012-05-23 15:04:18-0700 [-] [notifications] 2012-05-23 15:04:18-0700 [XmlStream,client] [twistedcaldav.notify.XMPPNotificationFactory#debug] RECV: <iq xmlns='jabber:client' id='H_7' type='result'><query xmlns='jabber:iq:roster'/></iq>

2012-05-23 15:04:18-0700 [-] [notifications] 2012-05-23 15:04:18-0700 [XmlStream,client] [twistedcaldav.notify.XMPPNotificationFactory#debug] RECV: <iq xmlns='jabber:client' from='pubsub.mycanonicaldomainname.com' id='H_8' type='error' to='com.apple.notificationuser@mycanonicaldomainname.com/icalserver.72bd94580b794782ac0eb9477c5dcef9'><error type='cancel' code='501'><feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/></error><pubsub xmlns='http://jabber.org/protocol/pubsub'><publish node='/Public/CalDAV/myrealdomainname.com/8443/'><item><plistfrag xmlns='plist-apple'/></item></publish></pubsub></iq>

That's not a very descriptive error, and according to XEP-0060 (the publish/subscribe draft for XMPP), it is also an illegal error, because it doesn't really elaborate on what "feature-not-implemented" means.

Has anyone run into this problem before, and if so, then what do I do to fix it?