ESP8266 freezes when UART is connected to D7 D8, where to go?

Hi,

I’m trying to connect multiple devices to the ESP8266 via the UART bus. On normal RX / TX pins, everything works fine. When I try to connect to a D7 (RX), the D8 (TX) ESP8266 hangs. I changed the contacts to D5 (RX), D6 (TX) - no data is transferred, the board does not hang. What could be the reason for the hang on D7 / D8? What other contacts can I use for the second device?

ESP8266:

#include <SoftwareSerial.h> // Serial Message Protocol

SoftwareSerial mySerial(D7, D8); // RX, TX (Serial for Periphery)

void setup() {
  mySerial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  delay(2500);
  mySerial.println("35");
  digitalWrite(LED_BUILTIN, HIGH);
  delay(5000);
  mySerial.println("30");
  digitalWrite(LED_BUILTIN, LOW);
  delay(2500);
}

Arduino UNO #1:

int message;
void setup() {
  Serial.begin(115200);
  pinMode(9, OUTPUT);
}

void loop() {
  if(Serial.available() > 0) {
    message = Serial.parseInt();
  }

  if(message == 30) {
    digitalWrite(9, LOW);
  }
  if(message == 35) {
    digitalWrite(9, HIGH);
  }
}