nRF52811 UART FIFO Communication Error

Hi,

I'm running a custom nRF52811 rev1 connected to a Quectel BC660K. I'm getting this error on RTT:

00> <info> app_timer: RTC: initialized.
00> 
00> <info> uart: Sending AT
00> 
00> <error> app: ERROR 4 [NRF_ERROR_NO_MEM] at src/bc660.c:124
00> 
00> PC at: 0x00019A4B
00> 
00> <error> app: End of error report

Here's the initialization:

#include "app_uart.h"
#include "app_error.h"
#include "custom_board.h"
#include "nrf_delay.h"
#include "system/timer.h"

APP_TIMER_DEF(RESPONSE_TIMER);

#define NRF_LOG_MODULE_NAME uart
#include "nrf_log.h"
NRF_LOG_MODULE_REGISTER();

#if defined (UART_PRESENT)
#include "nrf_uart.h"
#endif
#if defined (UARTE_PRESENT)
#include "nrf_uarte.h"
#endif
#define UART_HWFC APP_UART_FLOW_CONTROL_DISABLED
#define UART_COMMAND_BUFFER_SIZE     (48U)                /**< max number of test bytes to be used for tx and rx. */
#define UART_TX_BUF_SIZE 256                         /**< UART TX buffer size. */
#define UART_RX_BUF_SIZE 256                         /**< UART RX buffer size. */

#define ENTER_KEY 0xA
#define CARRIAGE_RETURN_KEY 0xD

bool init_bc660()
{
	uint32_t err_code;
	
	const app_uart_comm_params_t comm_params =
	{
		MAIN_RX_PIN,
		MAIN_TX_PIN,
		NRF_UART_PSEL_DISCONNECTED,
		NRF_UART_PSEL_DISCONNECTED,
		UART_HWFC,
		false,
#if defined (UART_PRESENT)
		NRF_UART_BAUDRATE_115200
#else
		NRF_UARTE_BAUDRATE_115200
#endif
	};
	
	APP_UART_FIFO_INIT(&comm_params,
		UART_RX_BUF_SIZE,
		UART_TX_BUF_SIZE,
		uart_event_handle,
		APP_IRQ_PRIORITY_LOWEST,
		err_code);
	
	if (err_code != NRF_SUCCESS) APP_ERROR_HANDLER(err_code);
	
	app_timer_create(&RESPONSE_TIMER, APP_TIMER_MODE_SINGLE_SHOT, uart_timeout_handler);
	
	if(send_init_commands() != NRF_SUCCESS) return false;
	
	return true;
}

And the uart event handler:

void uart_event_handle(app_uart_evt_t * p_event)
{
	uint8_t input_char;
	ret_code_t err_code;
	
	switch (p_event->evt_type)
	{
		case APP_UART_DATA:
			NRF_LOG_ERROR("UART Error: APP_UART_DATA");
			break;
		case APP_UART_DATA_READY:
			err_code = app_uart_get(&input_char);
			
			if (err_code)
			{
				NRF_LOG_ERROR("Error %d getting new character", err_code);
				reset_uart_input();
				break;
			}
			
			if (char_ends_message(input_char) || readBufferIndex >= UART_COMMAND_BUFFER_SIZE - 1)
			{
				app_timer_stop(RESPONSE_TIMER);
				append_char_to_buffer('\0');
				
				// Action
				NRF_LOG_INFO("Received: %s", readBuffer);
				response = true;
				memcpy(responseBuffer, readBuffer, UART_COMMAND_BUFFER_SIZE);
				
				reset_uart_input();
			}
			else
			{
				if (input_char != CARRIAGE_RETURN_KEY && input_char != ENTER_KEY)
				{
					append_char_to_buffer(input_char);
				}
		  
				ignoreNextEnter = false;
			}
			break;
		case APP_UART_FIFO_ERROR:
			APP_ERROR_HANDLER(p_event->data.error_code);
			break;
		case APP_UART_COMMUNICATION_ERROR:
			APP_ERROR_HANDLER(p_event->data.error_communication);
			break;
		case APP_UART_TX_EMPTY:
			// TX done. Will be called after successfully writing 1 character
			break;
		default:
			NRF_LOG_ERROR("UART Error: Code %d", p_event->evt_type);
			break;
	}
}

src/bc660.c:124 refers to APP_UART_COMMUNICATION_ERROR. What could be wrong?