Symptoms : When installing OpsMgr 2007 R2 web console you get error message for line 50 in the web.config file :

There is some KB article in Microsoft’s site about this scenario for web console installation over windows 2008. by posting this I want to mention that it occurs in Windows 2003 R2 SP1 too.
The solution is to download and install Microsoft Report Viewer Redistributable 2008