Cron is 403: forbidden with cron_key and through 'run cron' link
If token is valid (and it should be when you use Drupal-generated link), there is one other mechanism that can generate 403: .htaccess - check it for rewrites that might block it. If this will not help, try running cron from Drush. Drush bypasses Apache altogether, so you can know if the problem was Apache related (conf files, .htaccess), or purely Drupal related.