It's broken for a few day since my WP core was updated to 3.9

I’ve the same problem! Had to deactivate the plugin. I hope there will be an update soon!

This plugin is not the problem.
Smush.it seems to be very laggy : http://www.smushit.com/ysmush.it/

Hi everyone,

Thanks for posting on this! As @jgrmstr noted, Yahoo’s smush.it service has been intermittent lately, possibly getting a lot of traffic these days.

The issue’s been reported here:
http://wordpress.org/support/topic/http-error-36

For the moment, you can turn of auto-smushing at Settings > Media. Then you can do smushes manually whenever the service is available.

We’ll keep an eye on the service from here. 🙂

Cheers,
David