Can't use Arrow or Tabs keys to navigate ??

I have a situation with a protected spreadsheet where one particular user is
unable to use the Arrow or Tab key to navigate, whereas on my own PC the
Arrow and Tab keys work fine.

=> Any suggestions what could be the problem and how to fix it?

The sheet has only one macro as follows; its purpose is to protect the cells
while still allowing the outline [+] and [-] buttons to function:

Private Sub Workbook_Open()
With Sheet1
.Protect UserInterfaceOnly:=True
.EnableOutlining = True
.Outline.ShowLevels RowLevels:=1
End With
End Sub

{ Note: it may or may not be relevant, but the user is in Belgium, so most
likely he is using a foreign language version of Excel and/or Windows }