change config['log_path'] on the fly

This directive is not supposed to be changed at runtime, and you shouldn't try to.
Quite a lot of messages are logged before you ever have the chance to try altering the path.

Try this, it may need to be put into the CodeIgniter global space.

PHP Code:

$this->load->vars($this->config->item('log_path')); 


... and this cannot possibly help, in any situation. Totally unrelated.

I've lost count of how many times I've had to correct you for stuff like this, @InsiteFX. I know you're trying to help, but you can't just throw random suggestions, because when they don't work, you're doing the very opposite - people waste time with it and are often put in an entirely wrong direction.