The configuration file ci/config/config.php does not exist.

I see no reason for this to happen.

Might be just cache if the path didn't exist before ... Try putting clearstatcache(TRUE); in index.php (just once, you can remove it after one run), or look into OPcache - I'm not sure if it also caches stuff like this.

I found the error. my fault :-(
I've put the same requirement ci/config/config.php in autoload.php and based from the path it was not found.