New AdSense banner does not show and gives a 400 error
If you have recently created a new AdSense banner and it does not show on your web page and you are convinced there are no errors on your side, use Google Chrome's inspector feature to see if there are any errors loading the advert.
If you see the following error
Failed to load resource: the server responded with a status of 400 () then it's probably because the Google server has not finalised all of the information it needs to customise the banner delivery to your site. You won't see any errors on your website but Google's Chrome inspector will show that there has been an error in delivering the newly requested advert. [right click over your web-page and select inspect]
Put a known good banner back, leave it until the next day and then try again. You new banner will now display properly.
I had this problem especially with some of the new banner types such as Billboard. If you have the forethought, create the banners one day and then add them to your website the following day!