Unconformed wrote:2016-09-01 22:42:26 140344654886784 [ERROR] Fatal error: Can't change to run as user 'mysql' ; Please check that the user exists!
This looks like as you crippled your Xampp installation, as you overtook the contents of httdp.conf from your Linux Apache installation to the Xampp Apache installation and the my.ini for MySQL from Ubuntu to Xampp. This results in a mess. I dont think you will fix all that stuff, if i were you, i would delete the whole /opt/lampp folder and install a new one. And the do NOT simply override the default configuration. Take it, as it is.
You may also have destroyed the Phpmyadmin installation and/or the MySQL Data in the same way. It looks as everything has severe configuration issues.