Thanks for this. I want to use certsage because it makes it easy to renew the certificate. It was difficult (for me) to obtain and install the present certificate and I want to be able to use certsage on all of my sites so that the renewal process is uniform for all my sites.
While I'm honored that you wish to use CertSage across your enterprise, at present CertSage is not equipped with an installer for IIS, meaning that CertSage can acquire a certificate from Let's Encrypt for usage in IIS (once the IIS error when serving CertSage has been resolved), but you will need to install the acquired certificate by other means.
Assuming that your operating system for this server is Windows, I'm going to ping a couple of experts in this area to assist. They will possibly be able to offer you a better solution for Windows and IIS than CertSage can provide.
check IIS has your http bindings with hostname set to match your domain (it probably does): IIS Manage > Site > Bindings
In Certify The Web select New Certificate, select your IIS site, your domains (read from your IIS bindings) should be shown, if not check your bindings have names set in IIS.
Click Request Certificate, if that all works OK you should now have an automatically renewed https binding for each corresponding http binding you had originally.
If you're sticking with Certsage (which you've already had success with) you need to figure out the reason for the 500 error, this is likely caused by something in your configuration (see the link posted by @MikeMcQ ), I'd start by turning on error output.