Unfortunately, now I get this exception:
Mai 05, 2014 9:00:43 AM org.jivesoftware.smack.XMPPConnection$ListenerNotification run
SEVERE: Exception in packet listener
java.lang.NullPointerException
at org.jivesoftware.smack.util.LazyStringBuilder.length(LazyStringBuilder.java:59)
Well, at least we know that this can't be a side-effect of the now working login process. Somehow a 'null' value is added to LazyStringBuilder's List. If you enable assertions, then a exception is thrown onces this is tried. Please open a new thread if you need further assitance tracking down the root cause.