It's unlikely that it is anything to do with your ISP, that message will be coming from the Lufthansa server (it's what is known as a 403: Forbidden response from the server). The most likely cause is that the Lufthansa server has blacklisted your IP address for some reason. The simplest solution is to try to get a different IP address by rebooting your router. Switch the power off to your router, leave it a few minutes (the longer you wait the more likely it is you will get a different IP address) then switch it back on and wait for it to boot up again.
The web address you quoted actually re-directs to
https://www.lufthansa.com/gb/en/homepage Very few commercial websites now use http, most are https and will re-direct http requests to https.