Follow Jaroslav's suggestion.
The workaround was to physically power off the switch for 5 minutes, then power it back on.
A remote reboot does not help, it has to be physical.
After power on, the message disapeared and the switch works well.
Jaroslav is right even after 2 years.
Here is what I experienced.
1- Stack of 4 WS-C2960X-48FPS-L
2- Old IOS was c2960x-universalk9-mz.150-2.EX5.bin
3- New IOS that gave the ILET error - c2960x-universalk9-mz.152-2.E7.bin. Then we reloaded the stac...