When a user changes their email address, and 4images set to activate user by email or by admin, the user might get deleted from the database if some other visitor enter register page, before the user get activated.
Open register.php
Find two (2) times this line:
WHERE (".get_user_table_field("", "user_joindate")." < $expiry) AND ".get_user_table_field("", "user_level")." = ".USER_AWAITING;
Replace both lines with this:WHERE (".get_user_table_field("", "user_lastaction")." < $expiry) AND ".get_user_table_field("", "user_level")." = ".USER_AWAITING;