The bit you were trying to add on the bbcode page.
Something in that is triggering their mod_security. You need to work out which bit of it is doing that.
So put it in a normal test post on the board and that should also trigger the mod_security (if they turn it back on). Then what you do is just post half of it. If that half works, then the bit that triggers it is in the half you didn't post. You keep narrowing it down like that until you work out exactly what it doesn't like and they then to white-list that bit. That way you keep the security turned on, but the 'rogue' trigger word is cleared for use.