Resolved it by myself. I do not consider it to be the great solution I have discovered but I am answering this question that it might help someone who wants to get rid of this kind of issue and tired of finding the exact solution.
I have solved it by debugging skills with Trial and Error methodology (it requires so much patience in oneself).
What I did is, I started commenting some of the code that places dynamic controls on the page and checked for result whether the same error is generated or not? If yes, then you are commenting wrong code. So just remove the comments, make that portion as it was and move on.
At last I found that there were some dynamically generated tool-tips; I was used to display some additional details on every cell of the grid view (RadGrid), were creating this error. On PageInit I was clearing all of the controls of the division and adding a grid view by code behind. So each time the request gets initiated, it clears all of the controls and adds them newly.
However, the objects of tool-tip (I have used RadTooltip to display the tool-tip on cell) were not getting cleared from the page across the postbacks.
So I just applied a new ID to each tool-tip using its cell's (cell of grid) client id + DateTime.Now.Ticks.ToString()
, and I kicked off myself from this weird error.