ok, then it maybe a low memory problem. The default stacksize is 4 kB, try to start the threads with lower stack space. Jan’s F767 is a Jumbo compared to the KL25Z 
It can be set in the Thread constructor:
https://os.mbed.com/docs/mbed-os/v6.15/mbed-os-api-doxy/classrtos_1_1_thread.html#acd57cc8e3c47a4c0e400f80005426ade
or the defalt stack size can be changed, see Memory considerations:
https://os.mbed.com/docs/mbed-os/v6.15/apis/thread.html
but for a target with 16 k RAM I would use bare-metal profile and use Events. There you can schedule also cyclic activity.