Hmm, when the launch window pops up, asking you to enter your username and password, hit the options button at the top right corner of the window. Check the 'force verify' button and let it check all of the game files. This might help.
Tomorrow, they're doing extended maintenance on the game - maybe that'll fix the problem? Short of that, I think you may need to try an uninstall and reinstall of the game itself, rather than just the Arc client.
Hope you're able to get in :(