Google adsense responding the server responded with a status of 400 ()

I'm sure this issue is already solved. One of the possible reasons is that your ad unit is newly-created. Will need to wait for a while (hours perhaps) before you get the impressions.

It can mean:

• your ad is not approved yet or will be served soon
• the website is not yet approved to display ads
• the site has been disabled from serving ads

Check the status of your AdSense account and whether or not it's a host account. An "open" account doesn't necessarily mean it's been approved to show ads on a website. Source