It's indeed working now. Are you pulling that information from the blockchain.info API?
Not at all, we're 100 % independent. We run a full node dedicated for our server. Any others API providers or explorers can go offline, Goochain would be always up.
We also have developed our own BIP32/44 libraries to handle xpub searches and give out addresses list.
I'm looking for alternative (and maybe open source) blockchain explorers which fully support HD wallet lookup by xpub keys
First of all, Goochain is not a real block-explorer but a search engine. It gives you basic info and provides links to explorers. It can be seen as a "meta-explorer". Still, Goochain service can be very handy for quick basic check, as HD wallet amount for the current example. Goochain is light-weight and ad-free.
To be fair, I didn't even know BlockChain was providing this xpub feature. This is not documented at all from their side. I don't know any other website that provides this feature. I think some APIs service can have this feature (REST).
Antoine