If this is an issue on the remote web server, there's nothing you can do to resolve this. At best, a workaround might be available, pending a fix on the web server.
If multiple separate local computer systems are all showing the same error, then it's either with your router-firewall box or maybe a web content ("nanny") filter or an ad filter configured within that router-firewall box (not entirely common, but a few folks do have those configured in their router), or it's an issue with your DNS configuration (some folks have specific DNS server settings, and for any of various reasons including content filtering and ad filtering), or it's an issue on the remote web servers.
As for the app checks, you'll have installed a VPN client app, or an anti-malware app, if one or more of those apps are present. But if this misbehavior is showing across multiple devices, it's unlikely your iPad.
If it's not a sensitive website and if showing the error doesn't require a login onto the web site, you could post the URL address and somebody here might be able to independently check.