Identifying and solving UART error.

Error bits are cleared before HAL calls HAL_UART_ErrorCallback (just as status bits are cleared prior to calling the relevant callbacks). The error condition is stored in the ErrorCode member of the handle structure and will be some bitwise combination of:

/** @defgroup UART_Error_Code UART Error Code
 * @{
 */
#define HAL_UART_ERROR_NONE 0x00000000U /*!< No error */
#define HAL_UART_ERROR_PE 0x00000001U /*!< Parity error */
#define HAL_UART_ERROR_NE 0x00000002U /*!< Noise error */
#define HAL_UART_ERROR_FE 0x00000004U /*!< Frame error */
#define HAL_UART_ERROR_ORE 0x00000008U /*!< Overrun error */
#define HAL_UART_ERROR_DMA 0x00000010U /*!< DMA transfer error */
#if (USE_HAL_UART_REGISTER_CALLBACKS == 1)
#define HAL_UART_ERROR_INVALID_CALLBACK 0x00000020U /*!< Invalid Callback error */
#endif /* USE_HAL_UART_REGISTER_CALLBACKS */
/**
 * @}
 */

 If there's not an external pullup on the line, or if the ESP toggles it during startup, you probably have a frame error.

As I said, the error condition is stored within the ErrorCode member. Within the error callback, you would do something like the following to determine what error occurred, and handle it appropriately.

void HAL_UART_ErrorCallback(UART_HandleTypeDef *huart) {
 if (huart->ErrorCode & HAL_UART_ERROR_PE) {
 // parity error
 }
 if (huart->ErrorCode & HAL_UART_ERROR_FE) {
 // frame error
 }
 // etc...
}

Once you determine the type of error occurring, you typically would fix the source of that error so it doesn't happen again as errors should not occur during regular communication. But you could also clear the error and ignore it.