CMSIS-RTOS error: ISR Queue overflow on RTC2


we want to use RTC2 (RTC0 is used by SoftDevice, RTC1 by FreeRTOS) to keep track of time. I set NRFX_RTC2_ENABLED and NRFX_RTC_ENABLED in app_config.h. Code used for initialising RTC:
static nrfx_rtc_t _rtc = NRFX_RTC_INSTANCE(2);

void rtc_handler(nrfx_rtc_int_type_t type){
    if (type == NRFX_RTC_INT_COMPARE0)
        _time += _rtc_increment;

nrfx_err_t cal_begin(){
    nrfx_err_t err_code;

    //inititialize and enable target clock
    err_code = nrfx_clock_init(clockCallback);

    //low frequency clock is selected - check how to change source of the LF clock - RC, XTAL, Synth, ...
    while (!nrfx_clock_lfclk_is_running()) { /* wait until clock is running */ };

    nrfx_rtc_config_t config = NRFX_RTC_DEFAULT_CONFIG;
    config.prescaler = 4095; // 8 Hz
    err_code = nrfx_rtc_init(&_rtc, &config, rtc_handler);
    nrfx_rtc_tick_enable(&_rtc, false);

    err_code = nrfx_rtc_cc_set(&_rtc, 0, _rtc_increment * 8, true);

    return err_code;

cal_begin() is then called after nrf_sdh_enable_request in main.c. After 1 minute (when RTC handler is raised) DK stop execution, after some time we get error:

>         Error Status: 0x80020126 Code: 294 Module: 2
>         Error Message: CMSIS-RTOS error: ISR Queue overflow
>         Location: 0x20825
>         Error Value: 0x2
>         Current Thread: rtx_idle Id: 0x20003B6C Entry: 0x208D9 StackSize: 0x200 StackMem: 0x20003C38 
>         SP: 0x2003FF18 
>         For more info, visit:'
>     = System will be rebooted due to a fatal error =
>     = Reboot count(=5) reached maximum, system will halt after rebooting =

We also tried with no SoftDevice support, same result (error above). What can be done to solve this issue? Working with nRF SDK v15.0.0, mbedOS v6.6.


I’d advice using a debugger to find out the root of the error as you are mixing Mbed API with unmanaged nrfx calls. This is not something officially supported by Mbed OS.