Acrobat's "This file is already open" message

This is a really strange one. Our company has has an InDesign script that, near the end, calls Acrobat (by way of Applescript) to open a PDF file, replace a page in it, then save the file and close it. We've been using this script for over a year and a half now with no issues on 8 of the 9 computers we have. That last one, however, is giving me an odd message when it tries to open and save the file.

To be clear, all 9 computers are Macs, all running OS X 10.9.5 Mavericks. The script is on a central server, so they're all using the same file:

var unlockCover2014 = app.trustedFunction(function (fName, fPrefix)
{
    app.beginPriv();
    var folderPrefix = fName.match(/^.*?(?=JOBS)/);
    console.println("fName is " + fName);
    console.println("folderPrefix is " + folderPrefix);
    var myDoc = app.openDoc(folderPrefix + "Product Templates/ProofCoverNew/proof_cover_2014.pdf");
    myDoc.replacePages(0, fName, 0, 0);
    myDoc.saveAs(fName);
    myDoc.closeDoc(true);
    app.endPriv();
});

This file is stored in the correct folder to be a Folder-level script. 8 of the computers work through this without any trouble whatsoever. The 9th, however, puts this into Acrobat's Javascript console:

fName is /ArtDept/ArtDept/JOBS/425000-425999/425000 Folder/425000_cover.pdf
folderPrefix is /ArtDept/ArtDept/

RaiseError: This file is already open.
Doc.saveAs:9:
 ===> This file is already open.

I do not understand why this computer, alone, thinks that the PDF files are open already. The problem that arises from this is that, when the main InDesign script is done running, two documents are still open in Acrobat, and the one it's supposed to save does not get saved.

Any ideas about what's going on here?

I did finally discover what the problem was. I feel a bit silly about how (almost) obvious it is, but perhaps it might help others in my situation.

I disabled the line in the main InDesign script that was calling Acrobat, figuring I'd come back to that problem later after I dealt with some other issues. When I did so and ran the main script again, I discovered that Acrobat does, in fact, already open up a copy of that cover sheet PDF sometime during the execution of the main script! I was shocked, at first, but then I did a headdesk when I quickly realized the cause:

InDesign on this computer is set, by default, to automatically open a PDF after exporting it.

So, I just added a short line to the part of my code that sets the PDF Export Preferences to turn that feature off:

with (app.pdfExportPreferences)
{
    pageRange = proofRange;
    if (multiColor) pageRange = colorTable.toString();
    useSecurity = true;
    disallowChanging = true;
    disallowCopying = false;
    disallowDocumentAssembly = true;
    disallowExtractionForAccessibility = false;
    disallowFormFillIn = true;
    disallowHiResPrinting = false;
    disallowNotes = true;
    disallowPlaintextMetadata = true;
    disallowPrinting = false;
    changeSecurityPassword = "(NOPE)";
    viewPDF = false;
}

It's the viewPDF line at the end. (Sorry, I don't think I can highlight it with markdown.) I do feel silly that I overlooked such a semi-obvious cause, but I hope this might help someone else who is experiencing a similar issue. Thanks for trying to help anyway, @Loic.