1. Search for function _clean_input_keys on /system/core/Input.php
2. Modify this line
exit(‘Disallowed Key Characters.’);
to
exit(‘Disallowed Key Characters.’ . $str);
3. Refresh page to see the characters which generate the error
4. If you need to add those characters into the exception list, just add to this line
if ( ! preg_match(“/^[a-z0-9:_\/-]+$|/i”, $str))
I add | (pipe) character on the example above
Note: In most cases you can resolve the problem just by clearing the cookies