Misterious Run-time error 1004 on Workbook Open

I have two versions of the same file and they share the same sheet structure and the same workbook_open event function as follows

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for some reason one of the versions gives Run-time error 1004 when it arrives at line
ws.Visible = xlSheetVeryHidden

Run-time error 1004
method "Visible" of object "_worksheet" failed

but I cannot understand why. (in the other file with the same workbook_open event there are no issues)
ws is indeed each of the worksheets and I can access its properties through the inmediate window, but for some reason does not allow changing its visible attributes

any clues of what may be going on?

thanks!