adsense theme error message

Dear,
When I try to manage the ads from the plugin Adsense from Google I receive the following error message:
"Ads are not showing
Due to a theme change, ads are not being shown. Please reconfigure your ads or restore the previous theme."

The problem is that I never changed the theme and before I was having a problem with wordfence so I use to deactivate it when I want to modifiy the ads. But now I get this error also without wordfence.

Please can you help?
Thanks,
Paddy