SSL certificate verify failed

I have a gift voucher plugin for wordpress that creates a PDF after a user has finished filling in a form and sends it to the users email. This plugin uses the fpdf library to create the pdf.

It worked fine before my web hotel installed OpenSSL/1.0.2g. Now I think that the SSL certificate blocks the pdf creation. I'm getting the following error message:

PHP Warning:  fopen(): SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed in /wp-content/plugins/gift-voucher-pro/library/fpdf/fpdf.php on line 1259

And the code in fpdf.php on line 1259 is:

protected function _parsepng($file)
{
    // Extract info from a PNG file
    $f = fopen($file,'rb');
    if(!$f)
    $this-Error('Can\'t open image file: '.$file);
    $info = $this-_parsepngstream($f,$file);
    fclose($f);
    return $info;
}

My web hotel says that they have done everything correct and that I should contact the author of the plugin. But the author thinks that there is something wrong with the installation of the SSL certificate.

Does anybody here know what can I do about this?