@ cabdrome – To troubleshoot exactly what is causing the 500 Internal Server error do these steps.
1. Use FTP or your web host control panel file manager and delete the htaccess file in your website root folder.
2. Go to the htaccess Core >>> Custom Code page and cut (not copy) all of your custom htaccess code out of all of the BPS Custom Code text boxes and save all of your custom code to a Notepad or Notepad++ text file (not Word or WordPad).
3. Click the Save Root Custom Code button.
4. Go to the Security Modes page, click the Create secure.htaccess File AutoMagic button and activate Root folder BulletProof Mode.
If you do not see a 500 Internal Server error on your website then this means that none of the BPS standard htaccess code is causing the problem and you will need to isolate exactly which custom htaccess code is causing the problem by adding your custom htaccess code individually and testing it to isolate which custom code is causing the 500 error.
1. Add one block of custom code, save it, create a new master htaccess file with Automagic and activate Root folder BulletProof Mode. Do this for each block of custom code you have until you find out which code is the problem.
Also make sure that you are adding your custom htaccess code in the correct BPS Custom Code text boxes. Each Custom Code text box has a short description of which custom htaccess code goes in that particular Custom Code text box. The Read Me help button has some additional help information.
—————————
I checked your site and the Comodo SSL Certificate is valid and setup correctly. When I go to the https address the site displays correctly and the SSL Certificate info shows valid and setup correctly. Your site is not redirecting/rewriting to https from http. Are you adding the SSL rewrite htaccess code in the correct BPS Custom Code text box? Are you adding only this code shown in this forum topic and no other custom code in that text box?: http://forum.ait-pro.com/forums/topic/wordpress-ssl-htaccess-code-rewrite-ssl-rewritecond-server_port/#post-7233
Do all of the Custom Code steps very carefully again to make sure you are not making any mistakes.
————————-
I took a look at your site’s architecture and I see something that I have never heard of before IIS “Application Request Routing” and you have an Nginx / Apache mixed server environment (probably an Nginx Reverse Proxy). I think you will need to contact your web host to find out if the http to https htaccess rewrite code works on your server/website. Your host server may require some other htaccess code to do this on your server/website.
http://builtwith.com/cabdrome.com
Also you have W3 Total Cache installed on your site. You may need to remove all of the W3TC caching code from your htaccess file and then setup W3TC again after you have the http to https rewrite htaccess code setup correctly. I am not sure if you need to do that, but maybe?