Sharepoint client object model “File not found” Powershell

Question

Hello, I have some issue when i try to create a folder or a file with CSOM and PowerShell.

This is a sample of my code to create a folder :

$web_sp = $session_sp.Web
$session_sp.Load($web_sp)
$session_sp.ExecuteQuery()

$folder = $web_sp.GetFolderByServerRelativeUrl(($urlGed.Valeur+'/'+$ObjGed.Valeur+'/Commandes'))
$session_sp.Load($folder)
$session_sp.ExecuteQuery()

# at this moment if I check $folder.ServerRelativeUrl it returns the good url so I thinks it means it's loaded correctly

$myNewFolder = $folder.Folders.Add('NewFolder') # Here no exeption

$session_sp.Load($myNewFolder) #Folder or Folder.Folders
$session_sp.ExecuteQuery()

at the last ExecuteQueryan an exception has occurred ("File not found")
(When I use the Sharepoint web page I can create folder and file so I think I have the acces rigth