I think this might have to do with the WP Super Cache plugin if you have that installed. Here is what I did and it went away. FYI if you do a search on the error there are over 31 thousand sites affected. Anyway, the first thing I did, because I saw a lot of people suggesting it had to to with Akismet I deactivated and removed that plugin and installed Spam Free WordPress instead. That did not stop the error, so I am not sure that is the problem. So, then I looked at the WP Super Cache plugin and did the following:
1. Clicked on Delete cache from the setting page.
2. Deactivated the plugin
3. Re-Activated the plugin and reset the settings
4. Clicked on Delete cache again for good measure and the warning went away.
I am not sure if this works for everyone but these were the things that helped me, this is the only site I am seeing the problem on, and I don't have WP cache on all my sites, so cannot test if this is a solution that can be replicated.