After the most current patch many people have been having this problem after closing out of the game and starting it up again. This error is annoying because it will not allow you to hit play and start the game. But never fear there's a quick way to resolve this error.
You fix this error by going into the Tera folder and deleting the following two files:
Once the files are deleted the game will patch new versions of these files and then allow you to play the game once again. The issue's are related to Tera's servers so until they upgrade them or fix them this will be something we have to deal with. But it is worth mentioning how to temporarily fix it so you don't miss out on nexus or playing with your friends!