Soooo...odd one for ya here. I decided to do a short screencap, which I've attached to this post. Closing Steam and then running it as admin fixed the issue, but the fact remains that it happened in the first place, and, unbelievable as it may be, there are gamers out there in this world who do not know how to run a program as admin, or don't have an administrative account (underage, shared computer, etc.) and so can't run it as admin.
...I was a bit of a stooge and forgot to take a copy of the client log before starting the game post-Steam-as-admin, and it's now been overwritten with a log of the game working perfectly. Sorry.
I had just finished installing the game, and tried to run it; the issue happened immediately.