Joomla: 500 - An error has occurred

Ever reach a strange error like this ? when tried to login to the Administration section of my Joomla(1.7) with wrong username or password ?. 


"500 - An error has occurred - Return to Control Panel"


dont panic, just look into your configuration.php files, see what is under variable $log_path, and $tmp_path, and work by make sure the directory is writable by your webserver account ( ie apache user) or change it to different address that the server has with write permission. 


this problem occurred when we copy joomla installation to new server or for some reason, permission change.