The program cannot work correctly for more than "FFFFF"

[Ryan111]

Hi everybody
Today I was seeing and working around this project:

www.youtube.com/watch?v=HMxVDuk1gLo

There are two links in this youtube page to download the source code and executable program. you can download them.
after downlaoding it, I found out that it cannot work for more than "FFFFF". why?
I changed all of the numeric variables to quint64 to see if it would work for 64bit but still it doesn't work. I attached the changed source code. in your opinion what's wrong with this program?

[Lesiok]

What it means cannot work for more than "FFFFF" ?