During some installation of the VirtualMin panel you may encounter the error:

Connecting to software.virtualmin.com (software.virtualmin.com)|163.172.162.254|:443... connected.

ERROR: cannot verify software.virtualmin.com's certificate, issued by '/C=US/O=Let\'s Encrypt/CN=R3':   Issued certificate has expired.

Throughout this guide, we will roll a simple fix which would allow you to bypass the error. 

The error may occur upon downloading the installation file with the command: 

wget https://software.virtualmin.com/gpl/scripts/install.sh

In which case you may add the -no-check-certificate flag to wget: 

wget --no-check-certificate https://software.virtualmin.com/gpl/scripts/install.sh

However, upon running the install.sh you may encounter the same error. In which case you would need to reinstall the package ca-certificates of your server. 

To proceed on CentOS you would need to run the command: 

yum -y reinstall ca-certificates

On Debian / Ubuntu

apt-get --reinstall install ca-certificates

That's it. You may run the installation command again and it should be working flawlessly.