The remote server's SSL certificate has already expired - Plugin ID 15901

it should not matter if the certificate has a common name or not, i dont believe you cant create one without even. if the nessus scan only mentions the expired part and if you want to fix this, you could also just say we know. then just a new certificate with any common name should be fine. do keep in mind this certificate might also be used for the HA device trust. and with GTM it might be even more important.