I ran into this after updating a few 2960x’s with build dates of 2015. I attempted to down grade and had no luck with fixing. The research I did found 3 reasons the error popped up. Besides the error message popping up every 5 minutes or so, it killed all ip routing, making the switch useless. The exact error was:
%ILET-1-AUTHENTICATION_FAIL: This Switch may not have been manufactured by Cisco or with Cisco’s authorization. This product may contain software that was copied in violation of Cisco’s license terms. If your use of this product is the cause of a support issue, Cisco may deny operation of the product, support under your warranty or under a Cisco technical support program such as Smartnet. Please contact Cisco’s Technical Assistance Center for more information.
The two fixes are to shut down the switch, and leave it unplugged for 10 minutes. The third requires an RMA. I ran into the third one sadly. I even tried the latest iOS as of August 25 2019. You can read more about the error here: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-2960-x-series-switches/118837-technote-catalyst-00.html