I can suggest workaround on your own risk: remove this relation directly from database.
At first let's get know your certificate id. Go to this certificate and you can see its id in the address bar. For example:
/smb/ssl-certificate/edit/id/4/certificateId/3
Here certificate id is 3.
Finally execute following command via ssh but replace <CERT_ID> with your certificate id.
Code:
plesk db -e "delete from dom_param where param = 'webmail_certificate_id' and val = <CERT_ID>"