One of the VSTO MVPs pointed out that in some cases his customers were unable to resolve UNC paths consistently.  When he investigated further, he found that some branch offices of an enterprise were unable to resolve the UNC path ( \myservermyvstoappsinstallpath ) because of the way they setup their network infrastructure.  The only workaround he could find was to use a fully qualified web URL instead of a UNC path.  The resolution was to create a web server internal to the corporation which is still accessible by people in the branch office.  The other option would be to negotiate with the IT staff to change the way the branch office resolves server names, but apparently that was not an option.

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Use a path that resolves anywhere your VSTO solution could be deployed (Christin Boyd)