[SOLVED] Error while writing file to SD in Arduino Temperature logging station

I try to write a Temperature Logging station using a RTC 1307 and timestamping my measurements. The issue is at the beginning, creating the filename:

#include <dht_nonblocking.h>
#define DHT_SENSOR_TYPE DHT_TYPE_11

#include <LiquidCrystal.h>
#include <SD.h>
#include <Wire.h>
#include <DS1307RTC.h>

#define DATA_DIR "Temperature_Logs"
#define RECORD_FILE_EXT ".CSV"

static const int DHT_SENSOR_PIN = 2;
DHT_nonblocking dht_Sensor(DHT_SENSOR_PIN, DHT_SENSOR_TYPE);

int tempPin = 0; // Thermistor Pin

// LCD Display pins
//                BS  E  D4  D5  D6  D7
LiquidCrystal lcd(3, 4, 5, 6, 7, 8);

const int chipSelect = 9; // Set the chip select pin for the SD card
File logFile; // Create a file object

void setup() {
  Serial.begin(9600);
  lcd.begin(16, 2);

  Serial.println("Wait 1000ms to boot SD");
  delay(1000);

  // Initialize SD card
  if (!SD.begin(chipSelect)) { 
    Serial.println("SD Initialization failed!");
    while (1); // Halt
  }
  else {
    Serial.println("SD card initialized");
  }

  // Initialize RTC
  Serial.print("Initializing RTC...");
  setSyncProvider(RTC.get);
  if (timeStatus()!= timeSet) { // Check if the time is set
    Serial.println("Unable to sync with the RTC");
    while (1); // Halt
  }
  Serial.println("RTC is running.");

  // Create filename with current date
  tmElements_t currentTime;
  breakTime(now(), currentTime);
  String LogFileDir = "/" + String(DATA_DIR) + "/" +
                      String(currentTime.Year + 1970) + "-" + 
                      String(currentTime.Month, DEC) + "-" + 
                      String(currentTime.Day, DEC) + "_TemperatureLog" +
                      RECORD_FILE_EXT;

  Serial.println("Date: " + String(currentTime.Year + 1970)) + "_" + String(currentTime.Month) + "_" + String(currentTime.Day);
  Serial.println("LogFileDir: " + LogFileDir); // **Added for troubleshooting**

  if (SD.mkdir(DATA_DIR)) {
    Serial.println("Directory created");
  } else {
    Serial.println("Directory already exists");
  }

  // Verify SD card and file system
if (SD.exists(DATA_DIR)) {
  Serial.println("Directory exists");
} else {
  Serial.println("Directory not found");
}

  logFile = SD.open(LogFileDir, FILE_WRITE); // Open the log file
  if (logFile) {
    logFile.println("Timestamp, Temperature Thermistor (°C), Temperature DHT11 (°C), Humidity DHT11 (%)");
    Serial.print("Write header: ");
    Serial.println("Timestamp, Temperature Thermistor (°C), Temperature DHT11 (°C), Humidity DHT11 (%)");
    // Keep the file open for continuous writing
  } else {
    Serial.print("Error opening log file: ");
    Serial.println(LogFileDir);
    while (1); // Halt
  }
}

void logMeasurement(time_t t, float tempC, float temperature, float humidity) {
  tmElements_t currentTime;
  breakTime(t, currentTime);
  
  if (logFile) {
    String dataString = 
      String(currentTime.Year + 1970) + "-" + 
      String(currentTime.Month, DEC) + "-" + 
      String(currentTime.Day, DEC) + " " + 
      String(currentTime.Hour, DEC) + ":" + 
      String(currentTime.Minute, DEC) + ":" + 
      String(currentTime.Second, DEC) + "," + 
      String(tempC) + "," + 
      String(temperature) + "," + 
      String(humidity) + "\n";
    Serial.print("Datastring: ");
    Serial.println(dataString);
    logFile.println(dataString); // Using println to ensure newline
  } else {
    Serial.println("Error: Log file not accessible during measurement.");
  }
}

static bool measurement_environment(float *temperature, float *humidity) {
  static unsigned long measurement_timestamp = millis();

  /* Measure once every 10 seconds */
  if (millis() - measurement_timestamp > 10000ul) {
    if (dht_Sensor.measure(temperature, humidity) == true) {
      measurement_timestamp = millis();
      return (true);
    }
  }
  return (false);
}

void loop() {
  // Thermistor
  int tempReading = analogRead(tempPin);
  double tempK = log(10000.0 * ((1024.0 / tempReading - 1)));
  tempK = 1 / (0.001129148 + (0.000234125 + (0.0000000876741 * tempK * tempK)) * tempK); // Temp Kelvin
  float tempC = tempK - 273.15;
  float tempF = (tempC * 9.0) / 5.0 + 32.0; // Convert Celcius to Fahrenheit

  // DHT11
  float temperature;
  float humidity;
  if (measurement_environment(&temperature, &humidity) == true) {

    // Print to terminal with RTC timestamp
    time_t t = now();
    tmElements_t currentTime;
    breakTime(t, currentTime);
    Serial.print(currentTime.Year + 1970, DEC);
    Serial.print("-");
    Serial.print(currentTime.Month, DEC);
    Serial.print("-");
    Serial.print(currentTime.Day, DEC);
    Serial.print(" ");
    Serial.print(currentTime.Hour, DEC);
    Serial.print(":");
    if (currentTime.Minute < 10) Serial.print('0'); // leading zero
    Serial.print(currentTime.Minute, DEC);
    Serial.print(":");
    if (currentTime.Second < 10) Serial.print('0'); // leading zero
    Serial.print(currentTime.Second, DEC);
    Serial.print(" - DHT11 ");
    Serial.print(temperature, 1);
    Serial.print(" C, ");
    Serial.print(humidity, 1);
    Serial.print(" %; Therm ");
    Serial.print(tempC);
    Serial.print(" C, ");
    Serial.print(tempF);
    Serial.print(" F");

    // Display in LCD
    // Thermistor
    lcd.setCursor(0,0);
    lcd.print("Therm        C  ");
    lcd.setCursor(7,0);
    lcd.print(tempC);

    // DHT11
    lcd.setCursor(0,1);
    lcd.print("DH11     C     %");
    lcd.setCursor(5, 1);
    lcd.print(temperature,1);

    // Humidity
    lcd.setCursor(11,1);
    lcd.print(humidity,1);

    delay(500);
    // Log the measurement
    logMeasurement(now(), tempC, temperature, humidity);
  }
}

I've added outputs for the serial monitor to monitor what is going wrong: it seems to be the creation of the filename:

21:41:13.328 -> Wait 1000ms to boot SD

21:41:14.341 -> SD card initialized

21:41:14.342 -> Initializing RTC...RTC is running.

21:41:14.373 ->

21:41:14.404 -> Directory already exists

21:41:14.404 -> Directory not found

21:41:14.437 -> Error opening log file:

I've tried using AI to figure out the problem (Nemotron via HuggingFace) but it does not seem to find an issue with the code and now hallucinates a colleague named "miranda" who I should ask and gives me an agenda to talk to her.

Wiring should not be an issue since I tested the exact same wiring for SD card module with another program that just writes random data to a file on the SD card. This works fine and therefore should also disqualify any card formatting/power/hw issues