You may receive this error if you are running your site with impersonation.  Check your web.config impersonation tag and/or the security context that the particular user is running under:
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<identity impersonate="true" userName="UserName" password="Password" />

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In my case, the fix was changing the user that was being impersonated on my local machine.  I was able to pinpoint it to this issue because the site was working on my local machine with my development web.config, but not with my live web.config, and this was one of the main differences.  I didn't make it as far as determining what specific security access was different between the two users.  I tried granting my live web.config impersontated user full access to the bin folder on my local machine to no avail.