VBA code doesn't run after a while

We have an Excel 2007 (.xlsm) file which serves as an electronic logbook by several users.

When the file is open for some hours, some VBA code doesn't run anymore. Some facts:
-For instance a sub "selectionchange" which highlights the row selected doesn't run automatically anymore.
-If I try to step into the sub to check it step by step, the sub will not start.
-No error messages appear.
-Some users (different pc's) have the problem more often than other users.
-other VBA code (eg. macro's behind buttons) keep on working fine.
-macro's are enabled in the 'thrust center'

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    
    Dim icolor As Integer
    Dim cell As Variant
    Dim rij As Range
    Dim userFile As String

    Application.ScreenUpdating = False
    
    'On error resume in case user selects a range of cells
    On Error Resume Next
    icolor = Target.Interior.ColorIndex

    'Leave On Error ON for Row offset errors
    If icolor < 0 Then
        icolor = 36
    Else
        icolor = icolor + 1
    End If

    'Need this test in case Font color is the same
    If icolor = Target.Font.ColorIndex Then icolor = icolor + 1
    Cells.FormatConditions.Delete

    'Horizontal color banding
    With Range("b" & Target.Row, "f" & Target.Row) 'Rows(Target.Row)
        .FormatConditions.Add Type:=2, Formula1:="TRUE"
        .FormatConditions(1).Interior.ColorIndex = icolor
        Range("k12") = Range("g" & Target.Row).Value
        Range("l12") = Range("j" & Target.Row).Value
    End With

    With Range("l12")
        .FormatConditions.Add Type:=2, Formula1:=Range("l12") > 0
        .FormatConditions(1).Interior.ColorIndex = 40
    End With
    
    With Sheets("ingavescherm")
        Set rij = .Range("i12:i40")
    End With
    
    For Each cell In rij
        If cell.Offset(0, 1).Value > 0 Then
            cell.Interior.ColorIndex = 40
        End If
    Next


End Sub

Does anyone have an idea what can be the cause of the problem?

Thanks in advance!