SW-EK-TM4C123GXL: Using usb_dev_bulk example with a different UART from UART0

Hi everyone, 

I am trying to create an application where the TIVA is configured as USB device which communicates downstream through serial port. 

I am already tested with success the usb_dev_bulk example, where the pc sends to the USB device a string which is answered with CAPs. 

Now I am modifying this example to send the string received through USB from the PC to a a second device using an UART. There is no problem using UART0, buy I am not able to get it running with a different UART, even the use of a different one than UART0 seems to block the USB comunication. 

This is my UART configuration: 

//UART4
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOC);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_UART4);
    GPIOPinConfigure(GPIO_PC4_U4RX);
    GPIOPinConfigure(GPIO_PC5_U4TX);
    GPIOPinTypeUART(GPIO_PORTC_BASE, GPIO_PIN_4 | GPIO_PIN_5);
    UARTClockSourceSet(UART4_BASE, UART_CLOCK_PIOSC);
    UARTStdioConfig(4, 115200, 16000000);

An later in main after receiving from USB I just use the UARTSend function :

....

UARTSend(UART4_BASE, (uint8_t*)tx_buffer, 6);
....



//*****************************************************************************
//
// Send a string to the UART.  This function sends a string of characters to a
// particular UART module.
//
//*****************************************************************************
void
UARTSend(uint32_t ui32UARTBase, const uint8_t *pui8Buffer, uint32_t ui32Count)
{
    //
    // Loop while there are more characters to send.
    //
    while(ui32Count--)
    {
        //
        // Write the next character to the UART.
        //
        MAP_UARTCharPut(ui32UARTBase, *pui8Buffer++);
    }
}

The above code is working perfectly for UART0, but not for other UARTs. I think I have been very carefull with the GPIO, ports... configuration. 

Any ideas or possible reasons?

Best regards