There is one thing wrong with this code and another that may or may not be wrong with your server (but you didn't describe your problem so I cannot tell you for sure!!!!!!!!)
1. No admin normally has the flag ADMIN_ADMIN. This flag is only used by access() and cmd_access() (and other related functions). You should include amxmisc.inc and use is_user_admin(id).
2. You have the original Admin Chat plugin running.