Hello Max,
I think this is caused by the bug I reported as CONTRIB-3212.
This bug has been fixed but only very recently (yesterday or 2 days ago)
So most probably you have downloaded the certificate module before this bug was fixed.
But good news is that I think that downloading again from https://github.com/PukunuiAustralia/moodle-mod_certificate, and replacing the files with the new ones (be carefull not to overwrite any customizations you could have made to mod/certificate/type and mod/certificate/pix files) wil solve your problem.
Then go to Site Administration > Notifications and see if there is an upgrade needed (I am not sure there have been any change in database so maybe it will not be necessary to do any upgrade).
Then test issuing some certificates and see if the problem is solved or not. If the link is still wrong, please report on this forum becsause I want this nasty bug to be solved so I will make more tests !