SSL is enabled on Access Gateway server.
When a user is trying to access Apache Webserver based application via Access Gateway they are getting "Site can't be accessed" error.
Httpd error log contains the following messages:
[Mon Jun 01 15:50:44.900449 2020] [ssl:error] [pid 6672:tid 1776] [client XXX.XXX.XXX.XXX:38914] AH02039: Certificate Verification: Error (20): unable to get local issuer certificate
The problem was that a customer had SSLVerifyClient value set to "optional" on Apache side.
Release : 12.7
Component : SITEMINDER -WEB AGENT FOR APACHE
Set SSLVerifyClient to "optional_no_ca", or to "none" to completely disable it.