Public Function ErrorLog(objName As String, routineName As String)
Dim db As Database

Set db = CurrentDb

Open "C:\Error.log" For Append As #1
    
Print #1, Format(Now, "mm/dd/yyyy, hh:nn:ss") & ", " & db.Name & _
    "An error occured in " & objName & ", " & routineName & _
    ", " & CurrentUser() & ", Error#: " & Err.Number & ", " & Err.Description
   
Close #1
End Function
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Use it like this:

Sub MyExample()
On Error GoTo Err_Handler
' Your code here

Exit_Here:
    Exit Sub
    
Err_Handler:
    Call ErrorLog("frmMyForm", "MyExample")
    Resume Exit_Here
End Sub