Hello Jaroslav Cermak:
Could you let me know the silicon mask of both devices (the one that works and the one that fails)? You can see the mask number on the top of the MCU chip. The I2C module should work the same in both devices, but there might be an errata involved. Also MK60DN512Z is an old silicon revision of the K60DN512.
Also, please provide more information on the actual issue, so we can analyze the situation. What do you mean with "different behavior" and "both act weird"?
Regards!
Jorge Gonzalez