Uninstall both OS 5 and 6, restart the computer, reinstall OS 5, search for the vendor file, open it up in Notepad, edit the "Carrier name" in quotations and change it to your carrier, then save the file, close notepad. Now if you run apploader, it should put OS 5 back on your blackberry, as it now is set for your carrier. It worked for me and I had the same issue you had too.
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