You may notice that when you are navigating parts of the SMP console, you receive an error on the page. For example, Firefox gives a PR_CONNECT_RESET_ERROR error and MS Edge gives an ERR_CONNECTION_RESET error.
You will want to take note of the exact URL you are trying to access that throws the error. In this case, http://YourFQDN.com/Altiris/NS/Agent/AltirisAgentDownload.aspx
In IIS manager navigate to the same path: Sites\Default Web Site\Altiris\NS\Agent\AltirisAgentDownload.aspx
You may need to first navigate to \Sites\Default Web Site\Altiris\NS\Agent\AltirisAgent. Once there, click on Content View. Find the actual ASPX page, in this case AltirisAgentDownload.aspx, and click it. Finally click on Switch to Features View.
With the ASPX page selected, double click SSL Settings.
Make sure that Require SSL is not checked, and Client Certificates is set to Ignore.