[SOLVED] Form validation in 3.1.7

The error message came from CI's valid_email rule, not from my callback.
But today, the issue miraculously disappeared! Yesterday, I did a roll-back to 3.1.6 (I still had that folder on the server: system_316). On my local host, I couldn't reproduce the error on 3.1.7. So I renamed system back to system_316, and system_317 to system. I tried again, and now it's working.