Just a quick note: I had this very problem earlier this morning, despite never having it before. I tried it a few times with the suggested command line fix, but it did not change anything. What did help me was: Make a backup of the addcache.xml file in your Aion folder to be save. Open the original file with a text editor and change the content in both lines to -ip:126.96.36.199 -port:2106 -cc:2, now save the file and make it read-only. It seems that Aion somehow messes around with the file at every login. In addition enter the command line from above in your Aion launcher properties - not even sure thats needed with the read-only file, but was too lazy to try it out and it can't hurt. After i did the above i was able to log into the game fine, no problems anymore at all. Reminder: I play on a EU server, you need to change IP data for us. Maybe this helps some of you. Please remember you made these changes and revert them back after open beta.
On an open beta related note, NCsoft posted some Open Beta Updates to let us know they are working on the fixes. The official forums still contain very little to no information on the known infos tho. Looks like they're throwing out more beta keys as well.
Beside the login, crash, lag and Game Guard problems the game currently has to deal with there are also some wierd ingame bugs that annoy the hell out of me: If you use shield skills like eg. Shield Defense I your auto-attack is turned off sometimes. In addition the "new" left-mouseclick camera look is not behaving like we're used to and if you move at the same time while looking around some wierd behaviour occurs, like turning you into a different direction and such.
And remember: The Monster Raffle Contest is still going on!