It was an issue I ran into with the quick reply template. I had to do a lot of custom work to get our avatars the way they are (being big in your post and small next to the quick reply box). I made an error in a line of code that confused XenForo into trying to read a template that didn't exist thus the error with the quick reply/quote feature. It's not something that's going to be common among users because as I said it was an error on my part due to custom coding. For regular XenForo customers this shouldn't be a problem.