I had this same problem. I would reinstall XAMPP and be able to get to phpMyAdmin. Since everything was working fine, I would install my website files under /htdocs and then set the XAMPP folder to 777 for my website to have rights to write to its own files. As soon as I did this then phpMyAdmin would start with the issue. If you check your error log found in /Applications/XAMPP/xamppfiles/var/mysql you probably see something like this:
Warning: World-writable config file '/Applications/XAMPP/xamppfiles/etc/my.cnf' is ignored
It turns out the my.cnf file does not like the world to write to it and needs to have permissions of 600 set. Once this is done phpMyAdmin is all happy.