I'm sorry, I didn't know where to ask, but I cant' post replies on topics. I keep getting the error message even though I typed the right answer "You failed the spam check. Please try again!"

And obviously, I don't have this problem when starting a new topic

So you are talking about an issue with this forum, not with Limesurvey, right?

I would open a support ticket on lImesurvey.org. Us forum users (even those that have moderator rights like me), don't have control over this.