MSP430FR6989: Backchannel UART with ACLK

Dear, 

For my project I am using the backchannel UART connection of my MSP430FR6989 with the ACLK (not the SMCLK). I followed the examples of the example software, however, I don't seem to get it to work. 
My code is as follows: 

#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW | WDTHOLD;                 // Stop Watchdog

  // Configure GPIO
  P3SEL0 |= BIT4 + BIT5;                    // eUSCI_A1 UART
  P3SEL1 &= ~(BIT4 + BIT5);                 // eUSCI_A1 UART

  // Disable the GPIO power-on default high-impedance mode to activate
  // previously configured port settings
  PM5CTL0 &= ~LOCKLPM5;
  
  // XT1 Setup
  CSCTL0_H = CSKEY >> 8; // Unlock CS registers
  CSCTL2 = SELA__LFXTCLK | SELS__DCOCLK | SELM__DCOCLK;
  CSCTL3 = DIVA__1 | DIVS__1 | DIVM__1; // Set all dividers
  CSCTL4 &= ~LFXTOFF;
  do
  {
  CSCTL5 &= ~LFXTOFFG; // Clear XT1 fault flag
  SFRIFG1 &= ~OFIFG;
  }while (SFRIFG1&OFIFG); // Test oscillator fault flag
  CSCTL0_H = 0; // Lock CS registers

  // Configure USCI_A1 for UART mode
  UCA1CTLW0 = UCSWRST; // Put eUSCI in reset
  UCA1CTLW0 |= UCSSEL__ACLK; // CLK = ACLK
  UCA1BR0 = 3; // 9600 baud
  UCA1MCTLW |= 0x5300; // 32768/9600 - INT(32768/9600)=0.41
  // UCBRSx value = 0x53 (See UG)
  UCA1BR1 = 0;
  UCA1CTL1 &= ~UCSWRST; // Initialize eUSCI
  UCA1IE |= UCRXIE; // Enable USCI_A0 RX interrupt

  __bis_SR_register(LPM3_bits | GIE); // Enter LPM3, interrupts enabled
  __no_operation();

}


#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)            // 04/07/2020
#pragma vector=USCI_A1_VECTOR
__interrupt void USCI_A1_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_A1_VECTOR))) USCI_A1_ISR (void)
#else
#error Compiler not supported!
#endif
{
  switch(__even_in_range(UCA1IV, USCI_UART_UCTXCPTIFG))
  {
    case USCI_NONE: break;
    case USCI_UART_UCRXIFG:
      while(!(UCA1IFG&UCTXIFG));
      UCA1TXBUF = 0x55;
      __no_operation();
      break;
    case USCI_UART_UCTXIFG: break;
    case USCI_UART_UCSTTIFG: break;
    case USCI_UART_UCTXCPTIFG: break;
  }
}

Bit 4 and 5 of port 3 are being set correctly, I initialize the ACLK and connect it to UCA1, and the baud rate is calculated and put in. I also adjusted everything to use UCA1 and not UCA0. Howver it does not work. 

With kind regards and thanks in advance, 
Yannick