I loaded a CodeIgniter site today and received a blank page with just the message “Disallowed Key Characters”.
There are myriad of scenarios that can cause this to occur. In my particular example, it occurred due to the fact that my application/config.php file had the following set:
$config['cookie_prefix'] = "<CHANGEME>_";
I had put that in as a placeholder, but the “<” and “>” symbols are not allowed in this particular config value and throw this error. There are other symbols that will also cause this, so something to watch out for.
After you change this, remember to clear your browser cookies too.