AlarmBanner.SelectedAlarms(0).Server , allowing you to select multiple alarms from multiple systems and access the correct Server for each object in the array.
See the help for "Core Reference > Coding > Scripting > Common Uses for Alarm Banner Scripts" for some examples of this.
But if you call it from the script handler OnMessageReceived then I don't think there is any Server object as there is no context.
In your code do you have "On Error Resume Next" defined? My guess is an error or a VBScript Nothing is being returned so when you try and call the Override it is undefined.