When I create a task on Zoho Projects through API, the task is created successfully b ut sometimes the created task does not work in the Zoho Portal. 

I can see the task  added   in the correct tasklist, but when I click on it the screen turns black and then nothing happens. Also, I can not change the start date, duration, assignee, etc.

When I inspected this through Firebug, I saw that clicking on the task gives a 500 internal server error in the ajax request.

What could be the reason behind this ? If I am missing something in the API call, the task should not have been added, and if it is added it should work like other tasks that are manually added on the ZOHO Portal.