I tried Back, Refresh, Add Reply, Fast Reply, etc. it always does the same thing.
I didn't use anything out of the ordinary, unless M_o_n_t_y__P_y_t_h_o_n__s_k_e_t_c_h. without the underscores, of course, is considered somehow a character combination that's a potential security issue. I tried it just with those 3 words and the period at one point and still got the error. And only on that post.