Thanks for your kind reply. We have already fixed it by removing that Copy of Nolics dll. However, we are caught up with a NEW PROBLEM which is quite bigger.
We have created many page groups to maintain pages in a proper hierarchy. Before we encountered above exception, one of our main Page Group suddenly disappeared. We thought, it's just a fluke and when the exception got fixed, still that page group did not appear. We checked database and found that whole page group with 36 pages in it is no longer there any more.
None of us deleted that important page group. We even checked it it got moved to some other page group but in vain. Please throw some light on it. It's a great pain for us to build those pages again. Moreover, you offer page history but if a page or page group is not there itself, version history can't be used.
I am quite concerned if such thing ever happens again in future. So, can you please explain this situation to me?