SOLUTION:
Exectute the following SQL code in PHPMyAdmin... (Copy & paste)
This will change the log_id column to auto_increment... you get this error when it doesn't auto_increment!

Code:
ALTER TABLE admin_activity_log CHANGE COLUMN log_id log_id int(15) NOT NULL auto_increment;
Found in thread http://www.zen-cart.com/forum/showpo...5&postcount=13

Worked for me!

Hope it helps everyone else out there...