There are other solutions in that post, but it basically comes down to a bad upgrade. If the above doesn't fix your problem, can you let us know what version of Joomla you're using? Also, did you ever upgrade it from a previous version?Here's how to fix it:
I had the same issue:
1. Under Extension manager, click on Discover, select everything click Purge Cache
2. Under Extension manager, click on Manage, click on Refresh Cache
3. Under Extension manager, click on Update, click on Purge Cache
4. Click on Site > Control Panel
5. Hit CTRL + F5 (full refresh)
6. Now check it. It seemed to be a caching problem and although I'm not 100% sure which one I did a purge and refresh of them all and it quit showing me the message.
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