Error: "The attachment cannot be accessed the file may have been modified outside of Act!" when adding attachments to an Opportunity

Symptom: Error: "The attachment cannot be accessed the file may have been modified outside of Act!" when adding attachments to an Opportunity

Product Details: 
Product Family: Act!
Product: Act! Premium
Version: v18.1 Update 3

Environment:
OS: Windows 10

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Cause:
There is no known cause for this issue.

Solution: 

1. Close Act!
   
2. Reopen Act! and test issue