Pagination Problem 404 error when using Custom Permalinks

After changing the permalinks, everything works fine but when i click on page two of the pagination, it shows me 404 error.

http://www.examquizzes.com/online-quiz/science/
This link shows the list of post and it works fine.

But when i click on page 2, it goes to the following link and gives 404.
http://www.examquizzes.com/category/science/page/2/

However, http://www.examquizzes.com/online-quiz/science/page/2/ works fine. I cannot figure out why does wordpress changes “online-quiz” into “category” in the link which is creating a problem.

Just to add, my custom permalinks are
Custom Structure:
http://www.examquizzes.com/online-quiz-test/%postname%/

Category base: online-quiz

Please help !