We have an Airtime instance in a local network, without Internet access. After update (from 2.3.x to 2.5.1) Captcha started to appear, but as the server don't have internet connection it fails because cannot validate.
Although I do not recall seeing a setting for it in the GUI, There is a way to disable the captcha but it will require a couple of source file edits (and most likely will not be persistent across updates). First, edit lines 40-42 in airtime_mvc/application/controllers/LoginController.php from:
if (array_key_exists('recaptcha_response_field', $request->getPost())) {
This should stop airtime displaying and verifying the Captcha. WARNING: This code is un-tested at the very least make backups of the files being changed.