Hi,
i had the same issue. For me caching caused the trouble. I actually had an exception rule for cart an checkout in WPFC and Autooptimized so the issue didn’t come frome those plugins, but i lately tried out setting “define(‘WP_CACHE’, true);” in wp-config.php to see if that would further improve my performance. After deleting that rule, everything went fine again.
So as a solution you might want to make sure it is’nt an caching issue and that that rule is’nt activated in wp-config.
Hope it helps!
