I found that if I removed the word 'select' from the post, I could post it without any issues. Apparently, the system thinks that 'select' is a dangerous keyword to be present in the post.
I switched over to Apache web server and tried making the post without any changes - and the post was successful. It looks like Apache understood the content better than LightSpeed.
I've several questions -
1. Should I completely disable mod_security? Is it safe?
2. How do I make sure that mod_security is in place and yet users don't get nasty errors while making posts?
3. What's the best way to deal with such situation? Every time I face such issue, my web hosts folks whitelist the rule and we get going. But that's not a long-term solution.
So, the bottom line is - how do I ensure that my board works error free and accepts right content and rejects bad one?