Symptoms

Error when registering SSRS report as Service Instance
 

Diagnoses

When attempting to register an SSRS report as a Service instance, the error is thrown:

VALIDATIONCould not load file or assembly 'Microsoft.ReportViewer.Common, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.

One thing to note is that the K2 Blackpearl server and the reporting server are on two separate boxes.
 

Resolution

Installing the "Microsoft Report Viewer Redistributable 2005 SP1" and "Microsoft Report Viewer Redistributable 2008 SP1" on the K2 server and performing a server restart resolved the issue:

http://help.k2.com/onlinehelp/K2blackpearl/ICG/4.6.9/webframe.html_Prereqs_K2forReportingServices.html

Here are the links to download them:

RV 2005 SP1
http://www.microsoft.com/en-us/download/details.aspx?id=6219

RV 2008 SP1
http://www.microsoft.com/en-us/download/details.aspx?id=3841