The most common issue is the Let's Encrypt Certificate Authorities' certificates are not trusted.

There are multiple culprits possible including your firewall, intrusion prevention system, active directory certificate trust store and your local machine trust store.

If you get an error and bypass the certificate warning our css, javascript and images files may not load, this is because they are not served from the same subdomain as your site, you would also need to bypass the warning at www.pacescheduler.com to allow those resources to load.

You can find the Root and Intermediate Let's Encrypt certificates here: Let's Encrypt Certificates

For Microsoft products this documentation may be useful:

Consult the documentation or technical support for your various networking equipment on how to add certificate authorities to your devices trust store.

A good way to test is to check your site from your cellphone to see if it is the corporate network or an actual issue with our certificates.