I have the same problem. When a certificate is issued, the email attachment PDF looks fine, as does the 'sample' version from the edit menu. But when I click on the download button from the list of issued certificates, it just opens a blank browser window.
Update: something to do with permissions. My admin account can click on the same icon and it downloads a PDF, whereas the manager account (and teacher role) do not. This seems like a bug, as the permissions for "view a custom certificate" include both Teacher and Manager.
Note: the student is still able to download a PDF of their own certificate regardless of the above. This issue seems confined to teacher/manager.