This has happened about 3 times this last week

While I was doing leves playing as a THM, I suddenly could no longer cast anything. I would begin to cast, say, cure, my character would begin the casting animation, the cast bar would fill up, and...nothing. He would get stuck in the casting state and never cast the spell. I could cancel the cast and start another one, but he would get stuck on that one too. I could use auto attack, instant cast skills like Second Wind still worked, and enemies could still kill me, I just couldn't cast. I had to log off and on to fix it.