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.
Custom certificate (plugin)
Internal Zend error
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