Google AdSense adds are currently being blocked by Kaspersky anti virus and Internet security software.
This started sometime yesterday with their latest releases of their security program.
One AdSense publisher emailed Kaspersky to let them know and he received a reply "within minutes" saying:
Sorry, it was a false detection. It will be fixed in the next update.
Thank you for your help.
I assume they release updates often and the update will fix the issue soon.
Actually, it seems if you update the software now, it will resolve the issue. You just have to hope your readers update their software.
Forum discussion at WebmasterWorld.