ESP8266 I2C slave does not acknowledge data - c++

Question

I have a TM4C123 processor acting as a I2C master and a ESP8266 as a slave. For the ESP I am using the Arduino IDE with ESP8266 support installed at version 2.5.2, which should support the I2C slave mode. However, I can't get it to work. Even with the Arduino slave_receiver example, the slave does not acknowledge (ACK) the master's requests which I am displaying on a scope.
To make sure I am using the right address at least once, I implemented an address sweep on the master. And to make sure I am using the right pins on the ESP I implemented the master mode on the ESP first and did a pin sweep with an I2C slave device. So I am fairly certain neither of this can be the problem.
I am using the Olimex Mod-wifi board, with SDA Pin 12 and SCL Pin 13 (Schematic here)
Can someone help me on this? Here is my code:
// Wire Slave Receiver
// by devyte
// based on the example by Nicholas Zambetti <http://www.zambetti.com>
// Demonstrates use of the Wire library
// Receives data as an I2C/TWI slave device
// Refer to the "Wire Master Writer" example for use with this
// This example code is in the public domain.
#include <Wire.h>
#define SDA_PIN 12
#define SCL_PIN 13
const int16_t I2C_SLAVE = 0x12;
void setup() {
Serial.begin(115200); // start serial for output
Wire.begin(SDA_PIN, SCL_PIN, I2C_SLAVE); // new syntax: join i2c bus (address required for slave)
Wire.onReceive(receiveEvent); // register event
}
void loop() {
delay(1000);
Serial.println("Loop");
}
// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(size_t howMany) {
(void) howMany;
while (1 < Wire.available()) { // loop through all but the last
char c = Wire.read(); // receive byte as a character
Serial.print(c); // print the character
}
int x = Wire.read(); // receive byte as an integer
Serial.println(x); // print the integer
}

Answer 1

I had a compareable issue with the ESP8266. It was no problem sending data between the Arduino Nano (Slave) to the ESP8266 (Master) via I²C. But when I switch the modes (Arduino Nano = Master and ESP8266 = Slave) the Wire example doesn't work.
My workaround for this issue was to reduce the I²C working frequence from 100kHz to about 20kHz.