mainnet-rpc-node-0001.nkn.org is using CF to provide https certificate. We don’t set up any security challenge on CF, it’s just the default CF security mechanism. Basically CF thinks your IP address might be an attacker, that’s why you are seeing this challenge.
This is very rare, we didn’t see any single challenge on all client side or our server side. What IP address are you using the sdk, is it a VPS IP?
Basically if you are calling this from client side, the CF challenge should never show up unless CF is having a false alarm. If you are calling this from server side, you should not need https at all, in which case you can use
http://seed.nkn.org:30003 as seed node so CF is not involved. The (probably only) reason to use
mainnet-rpc-node-0001.nkn.org is to prevent browser blocking in https context.