I upgraded Joomla to 3.8 on a test site and CiviCRM (4.7.24) seemed to work fine. I've now replicated the upgrade on a live server (a different config'd machine but both PHP 5.6.31) and while it seemed ok at first, after logging in as admin, certain Profile Edit & all Contribution CiviCRM pages give an Error 500 (the default Name and Address profile doesn't work - but a custom profile does). Dashboard seems to load fine.
I've checked another Joomla 3.8/Civi 4.6.31 site and am getting the same problem. As I've got a working 3.8/Civi am going to try and figure what's different between the installs, but meantime, thought I'd post in case anyone has something similar or any ideas.