To unprotect my entire workbook on opening, I start on ThisWorkbook the
following procedure :

Private Sub WorkBook_Open()
Application.ScreenUpdating = False

For Each WkSht in Worksheets
WkSht.Unprotect Password:="mypassword"
Next WkSht

I have a lot of workbooks with this procedure fuctioning properly, however
there is one which each time results in Runtime error 57121 :
Applikcation-defined or object-defined error. Debugging points me to the line
: WkSht.Unprotect Password:="mypassword"

(In the same workbook I have a macro, using the same code, wich is working
without errors.)

Any idea how to solve this?