Not able to give approval in the form

Hi

I made a pdf form in livecycle designer where different user need to give their approval in the form. I searched in google and found an example here (http://assuredynamics.com/index.php/portfolio/using-hash-keys-to-lock-objects/). I changed 3 username given with the form and added 3 more username (By using Form properties>variables).

My problem is that when I want to give approval with user 5 & 6, I am getting below image

Sometimes I am getting below image when I try to give approval with User 1 to 4.

Here is the link of the form

https://drive.google.com/file/d/1x120UfFx900TNIwA0tkXexcz_5pe-jSL/view?usp=sharing

Can anyone please give an idea how to solve my problem.

thanks