How to use FEINT and OSTM1 resulting in unused_isr

I'm trying to use OSTM1 as a timer. Say I want to toggle a pin every x us.

OSTM1 is set as follow:

void R_OSTM1_Init(void){
/* Disable OSTM1 operation */
OSTM1TT = _OSTM_COUNTER_STOP;

/* Set OSTM1 control setting */
OSTM1EMU = _OSTM_DEBUG_STOP;
OSTM1CTL = _OSTM_MODE_INTERVAL_TIMER | _OSTM_START_INTERRUPT_DISABLE;
}

and started with:

void R_OSTM1_Start(void){
/* Enable OSTM1 operation */
/* Clear OSTM1 interrupt request and enable operation */
ECON_FEINTFEINTFC |= 0x00040000;
OSTM1TS = _OSTM_COUNTER_START;
}

on dr7f701649_irq.h:

#define FEINT_ENABLE 0x000000F0u
#ifndef FEINT_ENABLE
#define FEINT_ENABLE 0x00000000u
#endif

and on main.c I add:

#pragma ghs interrupt
void unused_isr(void){
    while(1){}
}

#pragma ghs interrupt
void FEINT(void){
NOP();
ECON_FEINTFEINTFC |= 0x00040000;
OSTM1_flag = 1;
}

I can debug through FEINT but as soon as it leave the function, it got stuck on unused_isr and i had no idea what interrupt is triggered.

Can anyone give me a hint, what went wrong?

Cheers

Arno