The Google Adsense Preview Tool has been bugging out for the past few days, only returning to normal just a day or so ago.
First reports came from Google AdSense Help forums on Thursday, February 5th. Google noticed the complaints on the 6th, and a Google representative said:
Thanks for bringing up the issue and sorry for the trouble. We've noticed the issue and are working on it. We'll inform you as soon as it is fixed. Thanks again for your understanding.
Then the following day, on the 7th, we received reports that the tool is now working again.
Forum discussion at Google AdSense Help.