phamdacloc on "Captcha result in 500 Internal error"

My site is running WP 3.8 using Explorable Theme. The site works perfectly fine, but after installing "Captcha" and "SI Captcha Anti-Spam", both of these plugins result in 500-Internal Error when I key in the INCORRECT captcha code on the post comments section.

Does anyone understand what could be causing this?

Thanks
Loc