I have an Internet (External) user with Windows Vista and (now) IE 9 with Office 2007 on her personal computer (not a member of the Domain). She has sufficient access to get to a specific SharePoint site we have in-house, and she has Full Control of a specific Excel document we posted in Shared Document Library. The site in IE is set as a Trusted Site and she can log in using her Domain credentials without a problem.
However, when she attempts to Check Out, she receives the following error: "This document could not be checked out. You may not have permission to check out the document or it is already checked out or locked for editing by another user.". If she tries to use Edit in Microsoft Excel, she receives: "The document could not be opened for editing. A Microsoft SharePoint Foundation compatible application could not be found to edit the document.".
It can't be a site permissions issue because I can take my Windows 7 laptop, connect to the SharePoint server from the outside Internet, log in as her and Check Out the same document, Edit it in Excel 2010, Save, then Check In. My laptop is also intentionally not a member of the Domain so I can test issues like this for our outside Engineers.
What could be preventing the ability for her to Check Out on her (personal) computer?