HCL Sametime 11.6 is supporting MongoDB 4.4 and this is the suggested version in case of new install.
During a Community setup is enable the Mongo security as described here , but the steps there are not enough, because 4.4 version in case of replica set.
This version require also the creation and the use of a shared key to every Mongo server involved in the replica set, even if the replica set is composed by 1 node.
The first step required is the creation of the authentication key that I made with the following command:
openssl rand -base64 741 > /etc/mongod.key
After the creation of the key, ownership and permission should be set in the right way: