I ran your code up to the refresh. Of course, since i don't have this dsn i
get a 1004 error with "General ODBC Error" . I don't have access to an
Informix db anymore so i can't help with Informixz specifically.

I am surprised the error doesn't say anything relating to ODBC. Have you
looked into the Application.ODBCErrors to get more details?
Dim err_code As ODBCError
dim appname as string
If Err.Number < 7 Then '7=Out of memory; when query cancelled
s = "Error(s):" & Strings.Chr$(13)
For Each err_code In Application.ODBCErrors
s = s & Strings.Chr$(149) & err_code.ErrorString & " (" &
err_code.SqlState & ")." & Strings.Chr$(13)
Next
If Err.Number < 0 Then
s = s & Err.Number & ":" & Err.Description
End If
Err.Clear
MsgBox s, vbCritical, APPNAME
ElseIf Err.Number = 7 Then
MsgBox "Query Cancelled by User (or Out of Memory).", vbCritical,
APPNAME
Err.Clear
Else
Msgbox "unknown error"
End If

What else has changed compared to your prior environment?
informix Driver?
Excel platform? While my QueryTable code used to work in xl97 , i had to
make changes on xl2k 'cause it was not working properly anymore. I had to
delete not only the QueryTables each time, and i had to delete the Named
Range given to the resulting range of the query (automatically given or not).
A few things like that, but cannot remember.

Sebastienm