Bad request (400) - POST /resto/oauth2/v1/token

piosid

After a couple of months, this endpoint suddenly stopped working, without any changes in code. The way that we create requests is below:

public async Task<OAuthTokenOutput> RefreshToken(string refreshToken)
{
    var basicAuthHeader = $"{_apiSettings.ClientId}:{_apiSettings.ClientSecret}".Base64Encode();

    var content = new Dictionary<string, object>
    {
        {"grant_type", "refresh_token"},
        {"client_id", _apiSettings.ClientId},
        {"client_secret", _apiSettings.ClientSecret},
        {"refresh_token", refreshToken}
    };
    try
    {
        var oAuthTokenResponse = await _lightSpeedApi.RefreshToken($"Basic {basicAuthHeader}", content);

        return oAuthTokenResponse;
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, "RefreshToken failed.");
        throw;
    }
}

Have any idea why it stops working, without changes in the code base?