SAP CRM IC Alert

HI, Experts!

I have configured IC Alert about opened service ticket when customer have been confirmed. When I test it, all is good, but on another computer with another user it's an error: "Failed to send alert message to browser" (Message class CRM_IC_ALERT, number 006) and Alert doesn't appear. This error message appear only once, on next customer confirmations we just have no alert. We use the same browser (IE9 in compatibility mode) and we have the same authorization. Why alerts don't work correctly?

Thanks!

Best regards,

Anton