Digital Signature Stops Macro

I have a new problem with the same code so have started a new thread. (Hope this is O.K. moderators?)

My code is conditionally opening spreadsheets in a folder, pulling information from certain cells and putting them in a master spreadsheet.

When it comes across a spreadsheet that has been digitally signed I get the error message-

Copy method of range class failed

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The debugger points at this line:

Cell.Copy DstRng.Offset(LR, C)

Is there a way to stop this happening?

If you follow the loop back, they increment the row count so the copied cells are pasted to the new spread sheet sequentially.

I really need to know why it stops working when it is trying to extract information from cells in a digitally signed file.