it is
For CR = 1 To UBound(ColumnKArray)

IT needs to be
For CR = 1 To UBound(ColumnLArray)

My bad. 


Option Explicit
 
Sub SamT_3()
    Dim Filename As String
    Dim NameLength As Long
     
    Dim FileNames As Variant
    Dim FileLines As Variant
    Const F As Long = 5 'CSV field number counting from zero
     
    Dim K_Array As Variant
    Dim L_Array As Variant
    Dim Sum_L As Double
    
    Dim Fn As Long 'Fn = Index number for FileNames
    Dim CR As Long 'CR = Column Arrays Index number. Same As Column Row number                                'j
     
    Const FolderPath As String = "C:\Users\dbrandejs\david\skola\IES\Diplomka\adjusted data\allstocks_20130102\" 'include ending \
     
     
     '''' Put all the file names in the path in Array
    FileNames = Filter(Split(CreateObject("wscript.shell").exec("cmd /c Dir " & _
    FolderPath & "*.csv /b /s").stdout.readall, vbCrLf), ".")
     
     
     '''' Open one file at a time
    With CreateObject("scripting.filesystemobject")
        For Fn = 0 To UBound(FileNames)
             
             'Initializ
            L_Array = 0
             
             ''''Put all lines from one file in Arrays
            FileLines = Split(.opentextfile(FileNames(Fn)).readall, vbLf)
            ReDim K_Array(UBound(FileLines))
            ReDim L_Array(UBound(FileLines) + 1)
             
             ''''Log(F) into Column K
            For CR = 0 To UBound(FileLines)
                K_Array(CR) = Log(Split(FileLines(CR), ",")(F))
            Next CR
             
             ''''Calculate Formula on Column K, put in Column L
            For CR = 0 To UBound(K_Array) - 1 '-1 to compensate for column formula offsets
                L_Array(CR + 1) = (100 * (K_Array(CR + 1) - K_Array(CR))) ^ 2
            Next CR
             
             ''''Sum of Column L
            For CR = 1 To UBound(K_Array)
                Sum_L = Sum_L + L_Array(CR)
            Next CR
             
             '''' Put results in sheet
             
             'Get FileName
            NameLength = Len(FileNames(Fn)) - InStrRev(FileNames(Fn), "\")
            Filename = Right(FileNames(Fn), NameLength)
             
             'Place result
            With ActiveSheet.Rows(Fn + 1)
                .Columns(1) = Filename
                .Columns(2) = Sum_L
            End With
             
        Next Fn 'Work on next File
    End With
End Sub