I was able to fix the issue. I don't know why this fixed it, but it did.

First I downgraded the following packages from 7.6.0 to 7.0.2:

• Microsoft.IdentityModel.Tokens
• Microsoft.IdentityModel.Abstractions
• Microsoft.IdentityModel.JsonWebTokens
• Microsoft.IdentityModel.Logging

That did not solve the issue on its own, but it is one action I took.

The key that seemed to solve it is that I removed the X-Frame-Options option from the web.config

<!-- Removed this -->    
<add name="X-Frame-Options" value="SAMEORIGIN" />

The X-Frame-Options tag was in

 <system.webServer>
    <httpProtocol>
    <!-- It was here -->
   </httpProtocol>
</system.webServer>

inside the web.config.

Taking those two actions made the site start and authenticate with SustainSys as expected.