Joomla: 500 internal server error on administrator panel.

The solution is rather simple.
You need to set the right permissions for log , cache, tmp and administrator/cache.
So goto your joomla root dir and

chmod 755 log cache tmp administrator/cache

Also make sure the file ownerships are correct.

chown -R myuser myjoomladir

And last to get rid of “500 – An error has occurred” while logging to the admin read the error_log which should be like this

[16-Nov-2013 10:00:37 Europe/Athens] PHP Warning:  fopen(/home/myuser/www/logs/error.php) [<a href='function.fopen'>function.fopen</a>]: failed to open stream: Permission denied in /home/myuser/www/libraries/joomla/log/loggers/formattedtext.php on line 248

Meaning you have to

chmod 777 /home/myuser/www/logs/error.php

Also one last thing to keep in mind is you need to have the correct path for log and tmp at your configuration.php file. Something like that.

 public $log_path = '/home/myuser/public_html/logs';
 public $tmp_path = '/home/myuser/public_html/tmp';

All set!