CURLE_WRITE_ERROR (23) in delete file from ftp server function using C++

CURLE_WRITE_ERROR (23) An error occurred when writing received data to a local file, or an error was returned to libcurl from a write callback.

My files from the FTP server are not being deleted because of this error. I don't understand what the problem is. If I remove the line "curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);", everything works fine—res will be CURL_OK, but the files will still remain in the FTP directory.

By the way, my FTP server supports the "DELE" command.

CODE:

int Ftp::deleteFile(const std::string& filename, const ServerInfo& server, const std::string& url)
{
    CURL* curl;
    CURLcode res;
    std::string fullRemotePath = url + filename; // url contains e.x. ftp://100.100.1.1/test.123

    if (!checkIfFileExists(server, fullRemotePath)) {  
        return -1;
    }

    curl = curl_easy_init();
    if (!curl) {
        return -1;
    }

    std::string deleCommand = "DELE " + filename; // ex. DELE test.123

    curl_easy_setopt(curl, CURLOPT_URL, fullRemotePath.c_str());  
    curl_easy_setopt(curl, CURLOPT_USERPWD, (wstringToString(server.login) + ":" + wstringToString(server.pass)).c_str());
    curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, deleCommand.c_str());
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    res = curl_easy_perform(curl);
    bool fileStillExists = checkIfFileExists(server, fullRemotePath); 

    if (res == CURLE_OK) {
        if (!fileStillExists) {
            logError(stringToWString("[FTP]: File successfully deleted: ") + stringToWString(fullRemotePath));
            curl_easy_cleanup(curl);
            return 1;
        }
        else {
            logError(stringToWString("[FTP]: The file was not deleted (error while checking): ") + stringToWString(fullRemotePath));
        }
    }
    else {
        logError(stringToWString("[FTP]: Error deleting file: ") + stringToWString(curl_easy_strerror(res)));

        if (!fileStillExists) {
            logError(stringToWString("[FTP]: However, the file no longer exists on the server.: ") + stringToWString(fullRemotePath));
            curl_easy_cleanup(curl);
            return 1;
        }
    }

    curl_easy_cleanup(curl);
    return 0;
}