wemos-d1-mini-esp8266-rfid-rc522.ino

/**

Useful links
https://wiki.wemos.cc/products:d1:d1_mini
https://cdn-images-1.medium.com/max/1400/1*YKc8KpAfMrlhrOLmNjdRwQ.png (D1 full pinout)
https://github.com/Jorgen-VikingGod/ESP8266-MFRC522
https://github.com/miguelbalboa/rfid

d1 mini rc52 wiring
https://discourse-cdn-sjc1.com/business5/uploads/mydevices/original/2X/e/ecedba79dc05f2c0b02b7fba8b3da2681590a11a.jpg
RST  - D3
MISO - D6
MOSI - D7
SCK  - D5
SDA  - D8
*/

#include "ESP8266WiFi.h"
#include <SPI.h>
#include <MFRC522.h>

constexpr uint8_t RST_PIN =  0;          // Configurable, see typical pin layout above 18
constexpr uint8_t SS_PIN =  15;         // Configurable, see typical pin layout above  16

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
  Serial.begin(74880);   // Initialize serial communications with the PC
  delay(1000);
  Serial.println("Setup");
  
  while (!Serial);    // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
  SPI.begin();      // Init SPI bus
  mfrc522.PCD_Init();   // Init MFRC522
  mfrc522.PCD_DumpVersionToSerial();  // Show details of PCD - MFRC522 Card Reader details
  Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
  Serial.println("Setup done");
}

void loop() {
//  Serial.println("Loop...");
  
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent()) {
    //delay(50);
    return;
  }

  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial()) {
    //delay(50);
    return;
  }

  // Show some details of the PICC (that is: the tag/card)
  Serial.print(F("Card UID:"));
  dump_byte_array(mfrc522.uid.uidByte, mfrc522.uid.size);
  Serial.println();
  
  // Dump debug info about the card; PICC_HaltA() is automatically called
  //mfrc522.PICC_DumpToSerial(&(mfrc522.uid));

  delay(1000);
}

// Helper routine to dump a byte array as hex values to Serial
void dump_byte_array(byte *buffer, byte bufferSize) {
  for (byte i = 0; i < bufferSize; i++) {
    Serial.print(buffer[i] < 0x10 ? " 0" : " ");
    Serial.print(buffer[i], HEX);
  }
} 

@ShaolinGardener I am seeing a similar things too. Have you solved your issue? :-)

Sorry guys, I'm really not doing these projects anymore and can not help there.
I remember I had issues with cheap ESP boards and MFRC522, so I usually order 2 from different sellers.

Firmware Version: 0xFF = (unknown)
WARNING: Communication failure, is the MFRC522 properly connected?
Scan PICC to see UID, SAK, type, and data blocks...
Setup done

I had the same error message.

-> Solved when I switched pins MISO and MOSI.