In pursuing an error a colleague of mine was having with
installing Data Quality Services for SQL Server 2012, I decided to test out my own installation of
DQS which had not been used in months only to run right snack into another issue with
DQS that I am sharing with you (plus a fix).
Data Quality Services utilizes the .NET Framework 4.0 with
use of its SQL CLR functions as part of the application. Upgrades to the framework on the host
machine can cause the potential for a mismatch in Module Version ID (or MVI)
for the .NET assembly that exists in the GAC.
Immediately upon launching the DQS Client, an error dialog
box would appear with the part of the error message saying:
Could not load or file assembly … or … Assembly in host
store has a different signature than assembly in GAC. (Exception from HRESULT:
0x80131050)
Full error message:

The good new is that there apparently is a fix already baked into the
product. You will have to run the
dqsinstaller from the command line with the –upgradedlls. Wish all fixes were this easy.
cd
C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Binn
dqsinstaller.exe –upgradedlls
Here are some references that were used:
http://social.technet.microsoft.com/Forums/pt-BR/a2060751-56c1-4c55-b880-8b9433dd447d/login-fails-logonwindowfailedtoconnect?forum=sqldataqualityservices
http://msdn.microsoft.com/library/hh479773.aspx