This error occurs in Excel or Word documents when the protected view settings change. This often occurs automatically when your Microsoft Office upgrades.
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To resolve this error please follow the below steps:

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1. Go to File, then select Options.

2. Select Trust Center, then select Trust Center Settings.

3. Select Protected View, uncheck all boxes.

4. Select File Block Settings, uncheck all boxes.

5. Select OK.

6. Restart Excel and re open the Excel or Word document.