The array is fixed or temporarily locked.

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I have pasted here an Excel VBA code snippet that fails on the indicated line. I am using Microsoft 365. Any assistance would be greatly appreciated.

Option Explicit
Option Base 1

Private Enum eCateg
   noObsolete = 0
   hasObsoleteSubfldrs = 1
   isOutOfDate = 2
End Enum
Private Type tyFldr
   pointr As Scripting.Folder
   categ As eCateg
   sw As Boolean
End Type

Private mFldr() As tyFldr

Private Sub FindObsoleteFolders(fldr As Scripting.Folder, isTopLevel As Boolean)
   Dim subfldr As Scripting.Folder, parnt As Scripting.Folder
   Dim protectedFldr As Boolean
   Dim m As Long
  
   If isTopLevel Then
      m = 1
      ReDim mFldr(1) As tyFldr
      With mFldr(1)
         Set .pointr = fldr
         .categ = hasObsoleteSubfldrs
         .sw = False
      End With
   End If
  
   For Each subfldr In fldr.SubFolders
      If isTopLevel Then
         protectedFldr = subfldr.Attributes = Hidden + System + Directory + Alias
      End If
     
      If protectedFldr Then
         protectedFldr = False
      Else
         m = m + 1
      '======== The following line runs fine when m = 2, but fails on m = 3.
      '======== Returns Err.Number = 10. Message reads, "The array is fixed or temporarily locked."
         ReDim Preserve mFldr(m) As tyFldr
         With mFldr(m)
            Set .pointr = subfldr
            .sw = isTopLevel Or .pointr.Name = "2023"
            If Not isObsolete(subfldr) Then
               .categ = noObsolete
               FindObsoleteFolders subfldr
            End If
         End With
      End If
   Next subfldr
  
End Sub

Private Function isObsolete(subfldr As Scripting.Folder) As Boolean
   'Loops thru a list of names of known obsolete folders, seeking match to subfldr.Name
   'Makes no reference to mFldr array.
End Function

Private Sub ProcessObsoleteFolder(fldr As Scripting.Folder)
   'Analyzes whether any file in the folder still has relevant data.
   'Makes no reference to mFldr array.
End Sub