tyeca how did you set up the permissions for "View forum" and "See past first post" ? Do you have any tag with custom permissions ("tag-scoped" permissions) or only global permissions (single column on permissions page)?
Are all non-admins users having issues or only some of them?
If everything looks good you could try disabling and re-enabling extensions to see if anything changes. If multiple extensions manipulate access permissions, the order the extensions are enabled in could have an impact.
