public $tmp_path = '/tmp'; public $log_path = '/var/logs';
and change to:
public $tmp_path = '/home/cpanelusername/public_html/tmp'; public $log_path = '/home/cpanelusername/public_html/var/logs';
Where cpaneluserame is replaced by your hosting accout userame (assumig the site is on a shared server, otherwise correct path).
If you still get administrator login error 500, make sure the file permissions are set to 755 on the /tmp and /logs folders, also /cache and administrator/cache/
Finally, the last cause of the 500 error at admin login attempt; this can be caused by a hosting account who's megabytes are full. If you've check the above and you're still getting administrator login error 500, check your hosting accounts disc space. You may need to ask for more room or delete some unused files.