After 20 fights you get a pop up with numbers you need to input. This was added as anti-bot system. You can't progress if you don't enter it. If you refresh it will pop up as soon as you log back in.
If entered wrong it remains on screen until the correct number is entered. So don't worry if you mis-tyoe as far as i know you won't get banned.
Note that Vip Level appears to have an effect on the fights before the pop up. It looks to be +5 more fight per Vip level.
As a method to prevent cheating during seasonal events. An additonal Captcha will pop up ever time before a boss is fought.
By clicking on the numbers on the Captcha you can change the numbers this will be randomly generated.