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I agree and really do hope that's not the case. (but then again world bosses and their 72h cooldow are equally absurd) Just wanted to share the information.

Some info was shared on the french forums, can't confirm or deny it but it makes sense (somehow):

It seems there is a seven day cooldown once the gate has been opened. Apparently once opened, you can still open it for the next six hours (reportedly the undieworld was accessible wenesday between 11pm and 5 am on Aerafal), but after that you have to wait one week to open it again.

That would explain why it was opened 3 times the first night on your server, but wouldn't open afterwards.

The requisite appears to be that all the candles are lit. And added to the latest info you posted -some candles extinguish by themselves- that means you need to be enough people to make sure all the candles are lit.

Again, can't confirm the info is accurate, but wanted to let you know where us frenchies were at  

There's no problem in having a puzzle that requires 20+ players to solve, but seriously, a puzzle without any ingame hint whatsoever, (or ingame feedback) that will only be solved by countless trial and error runs(or pure luck) is just poor and lazy design.
It's also a very cheap way to extend the new content's lifespan. (because honestly, a flat land, 15 candles, new mobs and a dungeon, woohooo)