How can I open a file in the SD card of a Marlin 3d printer and read its contents to a cha*?

Question

I need to write a custom function to add it to the Marlin (2.x.x) Firmware with the cardreader.h library (Anycubic Kobra max firmare) in order to read a gcode file and extract the comments in the beginning (thumbnail data). My code does not return any errors compiling, I know that the SD card is mounted, but in the serial monitor returns "Error opening file". I now the file exists, the name is correct but for some reason I cannot open it and read it.

here is the code:

#include "../../../../sd/cardreader.h"

char* readThumbnailComments(const char *filename) {
  if (!card.isMounted()) {
    SERIAL_ECHOLNPGM("Card not mounted!");
    return NULL;
  } else {
    SERIAL_ECHOLNPGM("Card mounted!");
  }

  char nonConstFilename[32];
  strncpy(nonConstFilename, filename, sizeof(nonConstFilename));
  nonConstFilename[sizeof(nonConstFilename) - 1] = '\0';

  card.openFileRead(nonConstFilename);

  char *result = (char*) malloc(492 * sizeof(char));
  size_t result_size = 492;
  result[0] = '\0';

  if (card.isFileOpen()) {
    char buffer[492];
    size_t index = 0;
    bool capture_comments = false;

    while (card.read(&buffer[index], 1) == 1) { // Read one character at a time
      if (buffer[index] == '\n' || buffer[index] == '\r') {
        buffer[index] = '\0'; // Null-terminate the string

        if (capture_comments) {
          if (strncmp(buffer, "; thumbnail end", 15) == 0) {
            break; // Stop capturing comments
          } else {
            if (index > 2) {
              strncat(result, buffer + 2, result_size - strlen(result) - 1); // Copy without the "; "
              strncat(result, "\n", result_size - strlen(result) - 1); // Add a newline character
            }
          }
        } else {
          if (strncmp(buffer, "; thumbnail begin", 17) == 0) {
            capture_comments = true; // Start capturing comments
          }
        }
        index = 0; // Reset the index
      } else {
        index = (index < sizeof(buffer) - 1) ? index + 1 : index;
      }
    }

    card.closefile(false); // Close the file
  } else {
    SERIAL_ECHOLNPGM("Error opening file");
    free(result);
    return NULL;
  }

  return result;
}

And from the UI menu cpp file I use following command:

 card.mount(); // Initialize the SD card
 char *thumbnail_comments = readThumbnailComments(filenavigator.filelist.longFilename());

Now if I print out thumbnail_comments it come out as null and lie mentioned in the console "Error opening file"

Any suggestions?