Could not load file or assembly

Issue: When users try to access Site Settings or Library settings, they see the above error message. 

This causes when the Microsoft.SharePoint.ApplicationPages.dll file is missing

Solution:  The Microsoft.SharePoint.ApplicationPages.dll file (found in C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\CONFIG\BIN) should be dragged into the GAC - Global Assembly Cache [C:\windows\assembly]. I did  this exercise on the Front - End and App Servers.

One you are done, recycle the App Pool of the Website you see the issue. 
Browse the Web Site >> Site Actions and now you will not see the above error message any more.