The captive.apple.com error suggests that you are trying to access the internet from a public WiFi hotspot; the URL is used by the iPad to detect internet connectivity. Errors relating to the address generally indicate a failure to authenticate the user via the hotspot operator’s paywall/terms page - prior to granting internet connectivity.
Perhaps this support page will be helpful with troubleshooting:
Use captive Wi-Fi networks on your iPhone, iPad or iPod touch – Apple Support
Apple reportedly uses this URL to test for a Captive Network:
https://www.apple.com/library/test/success.html
Are you selectively blocking any sites or protocols? You’ll need to check the setup of your WiFi Router. Pay particular attention to any firewall logs - as these may provide useful clues.