I’m at a loss… I’ve scoured the forums for this issue, and nothing that i’ve seen from anyone else accurately matches my problem. I’ve been trying to connect my wife’s 3ds and mine into a single room. This is something we’ve done without a single issue ever since MH4U became available. We took a break from it, and after some time, we’re getting back into it. Whenever either of us creates a room, the other simply cannot join, we can join other people’s rooms no problem, and anybody else can join rooms that we open no problem. I’ve even tried having both of us join someone else’s room with two open spots, and that works. Both of us have NAT type B, so its nothing that would indicate that our connection is overly faulty. I checked to make sure Luma and Nimbus are fully updated, which they are, and Pretendo is logged in on both 3ds. We can access other services, such as Miiverse perfectly fine. I’m honestly at a loss, and hoping that there is a good solution to help! 
Hmm I have no idea. Maybe try connecting to the server at delayed times? Like wait a minute or 2 before trying to connect?
I know it’s not helping anything at all, but me and my girlfriend have the exact same issue. We both have nat type B and can join other people’s lobbies no problem, it’s only when we try to join each other that we get 006-0612.
Others in Discord are reporting this too. I forward this to higher-ups. 
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Seems to have been fixed long ago.