Did you upload it by ftp? I would try uploading it again. Also make sure you use the one for your .NET framework, ie if you use .NET 4 make sure you upload the .NET 4 version of the file.Then if it still doesn't work try checking the permission on the file and make sure it is readable by the user that is the identity on the application pool.
Are you sure the folder you think you are looking at is the intended site/installation or are you sure the folder that you think is the root is the actual root folder? The error is very specifically saying it cannot find the file:
"System.IO.FileNotFoundException: Could not load file or assembly 'sts.MobileKit.Web' or one of its dependencies. The system cannot find the file specified.
File name: 'sts.MobileKit.Web'"
Also make sure there are no extra copies of web.config file in any folders besides the root folder. Sometimes people back up that file to folders like /Data and it can cause weird problems