Error 2147352567 (80020009) Could not proxy for user

The macro below to output Groupwise calendar items to Excel works fine on my
PC but fails with the above error message on others and I haven't the
faintest idea why.

Can anybody throw any light on it for me, please?

Thanks.

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Dim GWApp As Object ' Application
Dim GWAccount As Object ' Root Account
Dim GWProxyAcct As Object
Dim GWCalendar As Object
Dim GWAppts As Object

Private Sub cmdGo_Click()

' Clear old data
Range("A4").Select
Range(Selection, ActiveCell.SpecialCells(xlLastCell)).Select
Selection.ClearContents

' Start GroupWise session
Set GWApp = CreateObject("NovellGroupWareSession")
Set GWAccount = GWApp.login()

For i = 0 To 2 ' three proxy names
' Read Proxy name and get appointment messages collection
intRow = 4
strProxyName = ActiveSheet.Cells(3, i * 3 + 1).Value
Set GWProxyAcct = GWAccount.Proxy(strProxyName)
Set GWCalendar = GWProxyAcct.Calendar
Set GWAppts = GWCalendar.Messages

' Read dates
dateStart = Worksheets("CalSearch1").Cells(1, 2).Value
dateEnd = Worksheets("CalSearch1").Cells(2, 2).Value

' Read dates
For Each appt In GWAppts
'Output to Excel if start / end conditions met
If DateValue(appt.StartDate) = dateStart And _
DateValue(appt.EndDate) <= dateEnd Then
ActiveSheet.Cells(intRow, i * 3 + 1).Value = appt.StartDate
ActiveSheet.Cells(intRow, i * 3 + 2).Value = appt.EndDate
ActiveSheet.Cells(intRow, i * 3 + 3).Value = appt.Subject & "*" &
appt.FromText
intRow = intRow + 1
End If
Next
Next