I am using a STM32L052K6U6 to communicate with an SPI slave using the UART1 synchronous mode (configured with CubeMX, using the LL library). This is part of the code that receives data:

LL_USART_TransmitData8(USART1, 0);
while(!LL_USART_IsActiveFlag_BUSY(USART1));
while(LL_USART_IsActiveFlag_BUSY(USART1));
uint8_t data_1 = LL_USART_ReceiveData8(USART1);

Probing the bus with reveals that the clock generation and data sending works fine and the slave responds with the expected data. The receiver is enabled, but the data is not received by the microcontroller. The receive data register and the RXNE flag stay zero. What could cause this?