File transfer error

here is my problem about file transfer

as soon as i drag the file to be transfered, spark automatically post a retry button. the client will receive the file transfer invitation but fails when accepting it. here is the log.

Mar 23, 2009 11:46:25 AM org.jivesoftware.spark.util.log.Log error
SEVERE:
not-acceptable(406)
at org.jivesoftware.smack.Roster.removeEntry(Roster.java:293)
at org.jivesoftware.spark.ui.ContactList$19$2.run(ContactList.java:1606)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

got any idea on how to fix it? tnnx