Description:
When using captive portal with local user manager, any attempt to enter a voucher will result in "Invalid credentials specified". Doesn't matter if the voucher is valid or not.

Fix:
Add an isset() around the $config['voucher'][$cpzone]['enable'] in line 143 /usr/local/captiveportal/index.php

like so:

} else if (isset($config['voucher'][$cpzone]['enable']) && $_POST['accept'] && $_POST['auth_voucher']) {