AppleScript shows "Document could not be opened. You don’t have permission." but file can be opened?

Question

I am getting a "The document [filename] could not be opened. You don't have permission" when attempting to access several files in an AppleScript. However, I can open these files manually without issue.

I have attempted the following:

• Manually modified permission via File->Get Info
• Used Disk Utility to "Verify" and "Repair Permissions"
• Rebooted in recovery mode to reset home directory permissions and acls

I continue to have the problem.

To add further frustration, the files do not consistently report the error. Sometimes I will get the error on a file when the run the script, but not the next time!

Why might I be receiving this permissions error, and how else might I address it?

AppleScript below, if it helps:

-- prompt for source directory
set srcDirectory to (choose folder)

-- get list of all files in source directory
set allFiles to (list folder srcDirectory without invisibles)

tell application "OmniGraffle"
    -- create a new document
    set newDocument to (make new document with properties {template:"Single Pixel Grid"})

    -- added for debug purposes
    delay 5

    -- get a reference to the first layer
    set destinationLayer to first layer in first canvas of newDocument

    -- step through each of the file
    repeat with currentFile in allFiles
        -- get a reference to the next file
        set srcFileString to (srcDirectory as string) & currentFile
        set srcFileRef to (open srcFileString)

        -- get a reference to the icon
        set srcGraphic to first graphic in first layer in first canvas of srcFileRef

        -- flip the icon (they're all upside down)
        flip srcGraphic over vertically

        -- copy the updated source to destination canvas
        duplicate srcGraphic to destinationLayer

        -- close the source file
        close srcFileRef saving no

        -- added for debug purposes
        delay 5
    end repeat

end tell