Error: 500 Internal Server Error at Final Checkout

-- url of the page with the problem -- : www.catnets.com.au/
-- HikaShop version -- : 2.6.2
-- Joomla version -- : 3.5.1
-- PHP version -- : 5.4.39
-- Error-message(debug-mod must be tuned on) -- : Error: 500
Internal Server Error

Hi, I am getting a Error: 500 notice on catnets.com.au at the final step of checkout.

Please follow these setup to replicate the error.

1. Add a product to cart www.catnets.com.au/products/product/1-lo...tting-sold-per-metre
2. Click proceed to checkout
3. sign in as user: testuser / pass: pazzword
4. select any shipping & any payment & accept terms
5. click finish
6. then... Error: 500

I am also aware that and error log message would be helpful but either it is not producing one or the php settings on the server are not setup to do so, I had my server management look into it and their response was "Nothing is coming in the apache error log nor in php domain error logs.."

I have also tried the following with no change in the outcome:
* turned of SEF URL
* checked .htaccess
* change templates
* updated all software
* updated JoomlTools (EXTman & FILEman, this changed the notification from a 500 error to 0 error)
* turned off all modules
* turned off all HikaShop plugins
* moved the entire site to a different server
* uninstalled ALL 3rd party software (striped site down to bare-bone install)
* I have even uninstalled HikaShop and re-installed and the error still shows

I will keep testing, but any suggestions in the meanwhile would be great.

BINGO BONGO! Found the cause... but not the solution.

I had a backup of the site from about a week ago and tested it and it worked fine (no error message in final checkout). I did the update from Joomla 3.5.0 to 3.5.1 and it gave me an 'Error: 500 Internal Server Error" message, so something in the Joomla 3.5.1 update is causing it.

Aaaaaannnnnnnddddddd now I have found the dirty little sucker that waisted 8 hours of my life.

The updated /libraries/joomla/mail/mail.php file, once I replaced this with the 3.5.0 version the problem was fixed.