"Site Error" - Not able to do assignment

I have been trying to complete my assignments for 1 year now.
The "Site error" issue is happening very fequently now.

After i enter the assessment webpage and select a subject, I am hardly able to attend 2 questions.After which i get the following error:
Site Error:
This error has occured for one of the following reasons :
1. You have concurrently logged in using the same login.
2. You have used Back/Forward/Refresh button of your Browser.
3. You have double clicked on any options/buttons.
4. You have kept the browser window idle for a long time.
5. The server is too busy.
Click here to return to the login page.

I again login and continue the assignment and end up not ablt to attend more than 2/3 questions.This is happening for more than 3 months and how do complete the assignments with these issues.

After trying out for the 3 attmpts we are required to pay '150' to take up    
1 more attempt.Is this what for which the online assignment is introduced?

Regards,
Preethi