On a server running in an LXC container, the services such as Dovecot and MySQL will not start. The system logs are also full of permission denied errors.
cPanel & WHM is designed to run with unrestricted access to the operating system, and unprivileged containers often have limitations that can cause behavior differences and services to fail to start.
Run the server in a privileged container.
You can read more about unprivileged container limitations in our documentation here: Privileged vs unprivileged containers