nRF9160 modem hard fault debugging

Question

Hello, 
 
 My nRF9160 hard faults (i think?) when issuing a command to the modem, specifically this line: 
 query_modem("AT+CGSN", imei_buf, sizeof(imei_buf)); 
 which then calls ncs function: 
 /**
 * @brief Send a formatted AT command to the modem
 * and receive the response into the supplied buffer.
 *
 * @param buf Buffer to receive the response into.
 * @param len Buffer length.
 * @param fmt Command format.
 * @param ... Format arguments.
 *
 * @retval 0 On "OK" responses.
 * @returns A positive value On "ERROR", "+CME ERROR", and "+CMS ERROR" responses.
 * The type of error can be distinguished using @c nrf_modem_at_err_type.
 * The error value can be retrieved using @c nrf_modem_at_err.
 * @retval -NRF_EPERM The Modem library is not initialized.
 * @retval -NRF_EFAULT @c buf or @c fmt are @c NULL.
 * @retval -NRF_ENOMEM Not enough shared memory for this request.
 * @retval -NRF_E2BIG The response is larger than the supplied buffer @c buf.
 * @retval -NRF_EINVAL If @c len is zero.
 */
int nrf_modem_at_cmd(void *buf, size_t len, const char *fmt, ...); 
 
 This leads to I think is a hard fault and the board resets, the modem never returns any values, so I can't see what is happening. 
 NCS 1.9.1 
 mfw 1.3.2 
 
 What might cause this? 
 How can I debug this? 
 
 Thank you