Custom certificate (plugin)

Hmm, Zend is a PHP-interpreter.

Is that the complete Error Message you receive? I miss the reference, such as "Missing class information for blabla in ..."

The path C:\..\fpdf.php seems to be a mixup between the DOS-Drive C: and a relative path to the file (..\fpdf.php).

Maybe there is a mishap in the configuration.

Check where the file fpdf.php is located. You should also grep for the string ..\fpdf.php to see wich file expects the fpdf.php in a super folder.

Does this error occur with a certain certificate type only? Try a default one (portrait or landscape) and compare with your erroneous one.