Code 90000 = Lost connection with host.
Code 3102 = Account flagged as logged in at this data center.
These are normal error codes for any game (albeit on different numbers) 90000 literally is Diablo III's error 37.
Unless SE changes or removes the name for these codes, even after the game is addressed, you will encounter these.
Right now is launch stress. Everyone interested in the game is in a 'new game' craze and will be taxing the servers well beyond capasity.
This will be felt most keenly in the instance and transition servers, because they are bearing the full load of the entirety of the Data Center. The fact that this is happening in the EU/NA Data center is likely an infrastructure or hardware problem, not a software problem - which means this likely isn't a simple 'patch the game' solution.
All I can recommend for you guys is patience. I know it is a disappointment and many of you are very upset - bit as many have said, these sort of things are typical during the launch of an online games.
But none of this has a relation to the game itself, which should be where you are to take your judgement. Wait for the smoke to clear and play the game, then critique the game as you see fit.
Technical problems will be solved. It is design problems that may or may not change. That is where you should focus your feedback.
Thank you for your time.