I got this message as WARNING, and it looks to be related to this previous issue: JENKINS-12097

However, I can't get the plugin to work (version 1.28, im = 1.30, jenkins = 1.588 on CentOS 6.6).
Besides info saying it can't connect (but I see session opening for the user on the server side = eJabberd 2.1.13), this warning is the only thing I got in the logs to try to understand what's wrong:

Nov 10, 2014 2:49:51 PM hudson.plugins.jabber.im.transport.JabberIMConnection createConnection
INFO: Trying to connect to XMPP on /xxx.xxx.xxx
Nov 10, 2014 2:49:51 PM hudson.plugins.jabber.im.transport.JabberIMConnection setupSubscriptionMode
INFO: Subscription requests must be handled manually
Nov 10, 2014 2:49:51 PM hudson.plugins.jabber.im.transport.JabberIMConnection connect
WARNING: java.lang.ClassCastException: org.jivesoftware.smack.util.PacketParserUtils$2 cannot be cast to org.jivesoftware.smackx.vcardtemp.packet.VCard
at org.jivesoftware.smackx.vcardtemp.packet.VCard.doLoad(VCard.java:560)
at org.jivesoftware.smackx.vcardtemp.packet.VCard.load(VCard.java:542)
at hudson.plugins.jabber.im.transport.JabberIMConnection.vCardExists(JabberIMConnection.java:551)
at hudson.plugins.jabber.im.transport.JabberIMConnection.createVCardIfNeeded(JabberIMConnection.java:534)
at hudson.plugins.jabber.im.transport.JabberIMConnection.createConnection(JabberIMConnection.java:431)
at hudson.plugins.jabber.im.transport.JabberIMConnection.connect(JabberIMConnection.java:189)
at hudson.plugins.jabber.im.transport.JabberIMConnectionProvider.createConnection(JabberIMConnectionProvider.java:42)
at hudson.plugins.im.IMConnectionProvider.create(IMConnectionProvider.java:65)
at hudson.plugins.im.IMConnectionProvider.access$600(IMConnectionProvider.java:22)
at hudson.plugins.im.IMConnectionProvider$ConnectorRunnable.run(IMConnectionProvider.java:183)
at java.lang.Thread.run(Thread.java:745)

Nov 10, 2014 2:49:51 PM hudson.plugins.im.IMConnectionProvider$ConnectorRunnable run
INFO: Reconnect failed. Next connection attempt in X minutes

In ejabberd:

=INFO REPORT==== 2014-11-10 14:49:51 ===
I(<0.1484.0>:ejabberd_c2s:640) : ({socket_state,tls,

,<0.1483.0>}) Opened session for jenkins@xxx.xxx.xxx/36705839461415627391209190

Setting logs to FINEST did not help me so far...
But I never had this plugin working before. So I'm not sure I'm looking at a bug here.
Searching for one based on the WARNING just gave me this page.

Works like a charm on our test build server.
Thumbs up: this plugin is awesome.

This been said, you might have a closer look to what might happen when an error like this one is blocking.
Because it looks like a new connection is opened for each reconnect.
After a while, we've seen jenkins online with 6 sessions, even if we can't interact with any of them.
But I guess it could/should be another issue... Or not at all.

Thanks anyway.
Anything else to be tested before the next release?

Code changed in jenkins
User: Christoph Kutzinski
Path:
src/main/java/hudson/plugins/jabber/im/transport/JabberIMConnection.java
http://jenkins-ci.org/commit/jabber-plugin/87f188e7e7199e256c170008797b903f4c929f5a
Log:
JENKINS-25515 workaround for VCard parsing problem

Fixed in jabber-plugin 1.29

Regarding the session problem: if you have more info (e.g. thread dumps) please open an issue for this and I'll look at it. Low prio, since it doesn't seem to break anything.