Well, it appear problem came back.
I am running Varnish for caching, Pound and WP 3.6 not sure if any of this could cause the back-end to fail loading.
Apparently, reinsalling version 3.6 2 times one with an upload via SSH and one via the update button on the web-site fixed the issue.
I ignore why, i hope this will fix your issue as well.
