I have narrowed this issue down to the level of identifying that it's an InfoPath Publishing Error. Basically, I can raise it from the VS.NET when trying to deploy the K2 Process. I gave the user who encounters this problem an administrative permission on the site collection and still the same. I'm also wondering that it works fine on the Testing Server but in the live server it doesn't "I know the fact that everything works on testing environment where the live one is totally the opposite!".
However, I tested the Runtime Services authentication for that user and it's fine, the user can successfully access the web services. I've also reinstalled K2 Designer in that machine and nothing has been changed. From a different machine, I wasn't able to deploy the process "InfoPath Form" for that user. My username works fine in all levels and that certain user can't even detect if the given name of the MOSS library is correct or not.
Here is the usual error message:
Server was unable to process request. ---> Object reference not set to an instance of an object.
I'm still working on this issue and can't figure our what is missing... If someone has any tips or certain conditions to solve this issue, please let me know.