Right, \xE3\x83\x84 refers to U+30C4 (a Japanese character). We will update the forum software shortly, and then we'll also finally fix this problem, but for now leaving this character out of your post should help.
Strange, I just passed that string into a Python byte-array and decoded it. The result was this Japanese character: https://en.wiktionary.org/wiki/%E3%83%84. You currently cannot use this character.