Edited Unassigned: Could not load file or assembly [27798]

I have this error on every page of my application:

Could not load file or assembly 'AjaxControlToolkit, Version=15.1.1.100, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

I cannot find a solution for this problem.

Please, any clue is greatly appreciated.

Thanks