Change to your on CA issued certficiate on Nessus

Backup
cp /opt/nessus/com/nessus/CA/cacert.pem /opt/nessus/com/nessus/CA/cacert.pem.orig
cp /opt/nessus/var/nessus/CA/cakey.pem /opt/nessus/var/nessus/CA/cakey.pem.orig
cp /opt/nessus/com/nessus/CA/servercert.pem /opt/nessus/com/nessus/CA/servercert.pem.orig
cp /opt/nessus/var/nessus/CA/serverkey.pem /opt/nessus/var/nessus/CA/serverkey.pem.orig

Install

nessuscli import-certs –serverkey=nessus.qisec.se.key –servercert=nessus.qisec.se.crt –cacert=cacert.pem

[root@nessus ~]# /opt/nessus/sbin/nessuscli import-certs –serverkey=nessus.qisec.se.key –servercert=nessus.qisec.se.crt –cacert=cacert.pem
Saving a copy of the old key and certs and importing the new ones.
Confirming imported key and certs.
Successfully imported key and certificates.
[root@nessus ~]#