In our QA environment, Sitecore login page is always navigating to ?fbc=1 and bypassing the identity server. When I hit the identity server url directly, it gave me 500.19 error (above screenshot).
First, I started checking the event logs for errors and troubleshooting. As I am having Sitecore version 10.2, so I checked for all the pre-requisite required for it to verify anything is missing.
After some troubleshooting, I figured out that there is problem with IIS support, .NET core, .NET core runtime and URL rewrite. So, installing latest version of .NET core windows hosting bundle and URL rewrite 2.1 resolved the issue.
For more information on HTTP Error 500.19, check out https://learn.microsoft.com/en-us/troubleshoot/developer/webapps/iis/health-diagnostic-performance/http-error-500-19-webpage
Sitecore Practice 