I downloaded the beta previously, and played in the last two betas. My Tera folder contains 23 gigs. But when I started the launcher today, it began a huge download of some sort, after providing me with a message 'windows cannot find EME-something, etc.etc.'

Is this just a huge patch being added to the game, or is it downloading from scratch? If so, is that happening to anyone else?

Thanks in advance

I had the same issue, I just re installed TERA. But I don't know how to actually fix it besides the way I did it.
I would like to know a fix as well for future note.

It's probably the 4 GB OBT patch you forgot to DL earlier. And the error window just requires you to re-create the shortcut to the launcher. Just a bug.