Error with TCPDF

Hi,
I have a problem with TCPDF when I want to generate a certificate.

This is the error code :

Warning: Illegal string offset ‘url’ in /htdocs/wp-content/plugins/coursepress/2.0/include/tcpdf/tcpdf.php on line 18808

Warning: Illegal string offset ‘url’ in /htdocs/wp-content/plugins/coursepress/2.0/include/tcpdf/tcpdf.php on line 18808
TCPDF ERROR: Some data has already been output, can’t send PDF file

It’s impossible for me to see the preview when I set up the basic certificate and the same when a student want to access to his certificate.

Can you help me please ?

Thanks in Advanced.