What is a Sticky Element
Elements on a website which stick and remain visible. These elements will initially be in their position, and then in the event of scrolling down the webpage, their position will be following the scroll. An example of this is the secondary navigation on the home page of this site. See how it sticks to the top while the rest of the page scrolls.[/box]
We are not sure when these changes were made to AdSense policy. According to the current AdSense policy, you can not display AdSense ads in sticky elements.
Though there are some premium publishers who still show sticky AdSense ads, we highly recommend you to remove ads from sticky elements.
If you made the below nav ad unit sticky, you can disable that from design options ->Sticky elements.
If you are using AdSense ads in sticky sidebar, move them to non sticky sidebar or you can disable sticky all together as above.
Ref: AdSense Policy FAQ