Golang HTTP x509: certificate signed by unknown au

Question

I am creating a client app using Golang 1.9.2 and I am having some trouble to access my backend. The thing is that my app is working fine in the latest versions of Windows and Linux, however when I run it on Windows XP (yes, unfortunately I do have to support Windows XP, because some of our customers refuse to upgrade their OS) I get this error while trying to execute an HTTP GET and an HTTP POST: x509: certificate signed by unknown authority.

I've ran the same GET command using a Firefox ESR browser and a Chromium browser, from inside the Windows XP and none of them complain about the certificate.

Please note that my certificate is valid and signed by a trusted authority.

I've done some research and I found out that some people had the same problem and solved it by ignoring the TLS validation using this:

import ("net/http"; "crypto/tls")

tr := &http.Transport{
    TLSClientConfig: &tls.Config{InsecureSkipVerify : true},
}
client := &http.Client{Transport: tr}
resp, err := client.Get("https://someurl:443/)

So I added this to my code, but it is still not working:

// NewAPIClient - creates a new API client
func NewAPIClient() Client {
    c := &APIClient{}

    tr := &http.Transport{
        TLSClientConfig: &tls.Config{InsecureSkyVerify: true},
    }
    c.client = &http.Client{Transport: tr}
    return c
}

// GetTasks - retrieves a list of tasks from the backend.
func (c *APIClient) GetTasks() ([]byte, error) {
    conf := config.GetInstance()
    url := fmt.Sprintf("%s/myurl", conf.GetConfig().APIUrl)

    req, err := http.NewRequest(http.MethodGet, url, nil)
    if err != nil {
        log.WithError(err).Errorf("Error creating HTTP request")
        return nil, err
    }

    // Add headers
    req.Header.Add("Authorization", conf.GetConfig().APIToken)
    req.Header.Add("Accept", "application/json")

    log.Info("Retrieving tasks from the API")
    resp, err := c.client.Do(req)
    if err != nil {
        log.WithError(err).Errorf("Error retrieving tasks from the backend")
        return nil, err
    }
    defer resp.Body.Close()

    if resp.StatusCode != 200 {
        errMsg := fmt.Sprintf("Received status: %s", resp.Status)
        err = errors.New(errMsg)
        log.WithError(err).Error("Error retrieving tasks from the backend")
        return nil, err
    }

    tasks, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        log.WithError(err).Error("Error reading tasks response body")
        return nil, err
    }

    log.Info("The tasks were successfully retrieved")

    return tasks, nil
}

Is there a another way to solve this problem, without having to ignore the certificate validation? If not, what I am doing wrong in my code?