You don't say? I've been getting this message a lot over the past couple of days. I'd say roughly every 4th match or so I click engage and the dialogue box goes to zero and nothing happens. It's just a big tease! At any rate, when this happens it doesn't let me requeue for the match that I'm supposedly already in.

Is anyone else experiencing this too? How frequently?

Yes, I am experiencing it, and I think it's every time when a match didn't actually get off. I seem to remember that at least once I still got in a match later after trying to reqeue and getting this error message. (ANd I thin kit was without warning.)

I think about once or twice per evening (provided I play that evening.)

I've been seeing it most often when I change zones or something after the dialog box popped up. I think that when the game tries to move me to the arena I'm already moving to another zone, so it bugs out. Or maybe it just seems that way to me because I can't bear to sit in one spot for a minute.