The following are the possible causes for this problem:
Either the Instant Messaging server or the multiplexor is not running.
Incorrect multiplexor host or port names used in the Applet descriptor file (.jnlp or .html).
Different SSL settings used between Instant Messenger and the multiplexor.
Client and server version mismatch.
Where to get diagnostic information:
Instant Messaging server and multiplexor log files.
Instant Messenger logs.
Instant Messenger About dialog box, Details tab.