Recently, after some Microsoft .NET framework updates were applied to our SQL Server, one of our SQL Agent Jobs failed, with the error

System.IO.FileLoadException: Could not load file or
assembly 'System.DirectoryServices, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
or one of its dependencies. Assembly in host store has a different signature
than assembly in GAC. (Exception from HRESULT: 0x80131050)
See Microsoft Knowledge Base article 949080 for more information.</blockquote>

Upon further investigation, and reading the KB article, the solution turned out to be running the following ALTER ASSEMBLY command in the affected database.

ALTER ASSEMBLY [System.DirectoryServices]
FROM 'C:WindowsMicrosoft.NETFrameworkv2.0.50727System.DirectoryServices.dll'

References :

KB 949080
ALTER ASSEMBLY