Always keeps asking for activation code every login

Hello there!

Hope someone can help me. I wonder why if I login as different user, it always keeps on asking for activation code even if it has already been activated. I checked the Users table in the backend, the active status doesn’t change from No to Yes. What seems to be the problem. Please help. Thank you.

http://ww.wp.xz.cn/extend/plugins/user-activation-email/