It seems that it is impossible to use at the same time the PWMout (mbed API) and timer interrupts (Ticker or RTOS) or InterruptIn in the KL25Z, is there any idea to fix it?
KL25Z should have PWM as periphereal. It requires initialization and start command. It does not require anything from the mcu core.
Check that you are not using timer or interrupt which is connected to the same port you are using for PWM.