I’m using libcurl for FTP. My code seemed to be running fine and files were uploaded as I expected. But after a while, I found the connection went wrong and the error message is here.

FTP Folder Error:
Windows cannot access this folder. Make sure you typed the file name correctly and you have 
permission to access the folder.

Details:
The connection with the server was reset.

I asked my colleague to help fix the server yesterday and my code could upload files again. However, this problem occurred again this morning and I have to wonder if there is something wrong with my code. Here is my code.

#include <iostream>
#include <curl/curl.h>

bool createFtpDirectory(const std::string& url, const std::string& userpwd) {
    CURL* curl;
    CURLcode res;
    bool success = false;

    curl = curl_easy_init();
    if (curl) {
        // Set the URL for the FTP operation
        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        // Specify the username and password for the FTP server
        curl_easy_setopt(curl, CURLOPT_USERPWD, userpwd.c_str());
        // Specify the FTP command to create a directory
        curl_easy_setopt(curl, CURLOPT_FTP_CREATE_MISSING_DIRS, CURLFTP_CREATE_DIR);
        // Perform the request
        res = curl_easy_perform(curl);
        if (res != CURLE_OK) {
            std::cerr << "curl_easy_perform() failed: " << curl_easy_strerror(res) << std::endl;
        }
        else {
            success = true;
        }
        // Clean up
        curl_easy_cleanup(curl);
    }
    return success;
}

bool uploadFileToFtp(const std::string& url, const std::string& userpwd, const std::string& localFilePath) {
    CURL* curl;
    CURLcode res;
    FILE* hd_src;
    bool success = false;

    // Open the file to be uploaded
    errno_t err = fopen_s(&hd_src, localFilePath.c_str(), "rb");
    if (err != 0 || !hd_src) {
        std::cerr << "Failed to open file: " << localFilePath << std::endl;
        return false;
    }

    curl = curl_easy_init();
    if (curl) {
        // Set the URL for the FTP operation
        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        // Specify the username and password for the FTP server
        curl_easy_setopt(curl, CURLOPT_USERPWD, userpwd.c_str());
        // Specify the read callback function
        curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
        curl_easy_setopt(curl, CURLOPT_READDATA, hd_src);
        // Perform the request
        res = curl_easy_perform(curl);
        if (res != CURLE_OK) {
            std::cerr << "curl_easy_perform() failed: " << curl_easy_strerror(res) << std::endl;
        }
        else {
            success = true;
        }
        // Clean up
        curl_easy_cleanup(curl);
    }

    fclose(hd_src);
    return success;
}

int main() {
    const std::string ftpUrl = "ftp://MY_FTP_PATH";
    const std::string ftpFileUrl = "ftp://MY_FILE_PATH/template.xlsx";
    const std::string userpwd = "USER:PWD";
    const std::string localFilePath = "template.xlsx";

    // Create directory on FTP server
    if (createFtpDirectory(ftpUrl, userpwd)) {
        std::cout << "Directory created successfully." << std::endl;
    }
    else {
        std::cerr << "Failed to create directory." << std::endl;
        return 1;
    }

    // Upload file to FTP server
    if (uploadFileToFtp(ftpFileUrl, userpwd, localFilePath)) {
        std::cout << "File uploaded successfully." << std::endl;
    }
    else {
        std::cerr << "Failed to upload file." << std::endl;
        return 1;
    }

    return 0;
}

I hope someone can point out the loopholes in my code, or give me some other ideas to solve this problem.
Thanks in advance to everyone who sees this question.