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The app 'reads' all tasks through the lens of a Scope Owner. A Box can only include tasks that to which the Scope owner has at least viewing access to.
In the example below, "Tofu" is the Jira user chosen to be a scope owner. User "Tofu" doesn't have permission to access the project added to the scope definition. You as As a logged-in user, you can have full access to a given item, but if the Scope Owner can't access it, the error will pop up. 
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Solutions
Change the scope owner
You can use a different user as a scope owner. The new Scope Owner user has to have permissions allowing for access to must have permission to access a given project/filter/board.
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In the example above, the Project permissions were the problem. Go to project settings:
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You have two options.
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Associate a different scheme with the project.
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Modify the existing permissions scheme.
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