Our company houses external-facing collaboration sites for our third party users. We recently 3-4 months ago, updated to SharePoint 2010 and since then have had some issues with our external users not being able to access the sites. We assign (onboard) a user to a specific group (example: co name_readers, co name_contributors) for security reasons and to make it easier to manage the users and the sites. Since the upgrade, some users in a specific group can no longer access the main collaboration site. I thought, at first, that it might be a permissions issue but others in that same group (and at the same company) ARE able to access the site. I have filed trouble tickets with our IT dept on numerous occasions without resolution. They get an access denied message. Is it possible there is some setting from the user side that would prevent them from accessing the SharePoint sites? I don't know how to advise our users and am at a total loss. Note: I have asked users to try accessing with different Internet Explorer versions, thinking that might be the issue, but they are still unable to access.
If there is any custom branding, make sure that all the users have access to those resources, they are checked in and approved. This could result in an access denied error.