I have the same issue,
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.14) Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8 Lightning 1.0b2 Provider for Microsoft Exchange 0.12
I just turned off "Show missed Alarms " setting, and seems work, but when I have a new alarm, it shows missed alerts in the dialog.
I have the same issue,
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.14) Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8
Lightning 1.0b2
Provider for Microsoft Exchange 0.12
I just turned off "Show missed Alarms " setting, and seems work, but when I have a new alarm, it shows missed alerts in the dialog.