Custom certificate (plugin)

Hello,

We just migrated to a new server and I tried to generate a certificate, but I'm getting this error:

Warning: fopen() [function.fopen]: URL file-access is disabled in the server configuration in /home/public_html/lib/fpdf/fpdf.php on line 1522

Warning: fopen(http://server/mod/certificate/pix/signatures/sig.png) [function.fopen]: failed to open stream: no suitable wrapper could be found in /home/mycourses/public_html/lib/fpdf/fpdf.php on line 1522

FPDF error: Can't open image file: http://server/mod/certificate/pix/signatures/sig.png

Any ideas? If I go straight to the URL, the image comes up fine, so I'm having trouble figuring out what this error is referring to. We are using 1.9.4+ (Build: 20090225).

This red line is line 1522:
function _parsepng($file)
{
//Extract info from a PNG file
 $f=fopen($file,'rb');
if(!$f)
$this->Error('Can\'t open image file: '.$file);

TIA,
Brian