never heard it...
Exit update-notifier, and try in a terminal
update-notifier > $HOME/crashlog_unl.log 2>&1Is update-notifier started, so open synaptic with it, maybe we see what goes wrong.
If update-notifier again crashed synaptic, send me the log file, you find it in your home folder as crashlog_unl.
EDIT: Do not post the crash file here in forum, if you are a pass user