Today I came across one of my sites which for some reason was displaying the homepage OK, but when I clicked on the category view (or any post for that matter) it returned an error 403.
I googled and tried a few things, but in the end I solved it by changing the file permissions on my public_html folder using an FTP client.
Somehow, the folder had been set to 711 permission. I changed it back to 755 and that solved it. Also, just check the permissions for index.php – that will need to be 755 too.
Hope this helps someone!