Hi slo,
i had nearly the same problem. I'm using CW 10.3 and a TWR-K60N512, too. On the CPU package is the writing PK60N512VMD100 (Kinetis K series, 100 MHz, 144 pins).
If i am using in PE the CPU MK60DN512ZVLQ10 (with Z), everything works fine.
But if am using the CPU MK60DN512VLQ10 (without Z), i always get the unhandled interrupt too.
My only advice is to use the CPU with the Z part and do not care about it.