You're calling the square function slightly wrong (you're passing a vector containing a number, and not a number; watch the brackets), but I actually get a different error than you (java.lang.ClassCastException: clojure.lang.PersistentVector cannot be cast to java.lang.Number).
It sounds like your TryClj session might be hosed, or the site was having issues at the time? Maybe try again.
It evaluates fuzz then it evaluates 8. The former will either resolve to a symbol and it's internal name will be displayed (that sandbox...$fuzz... string you saw) or an exception if it is unknown. The latter will resolve to the value 8 which will be displayed.