Could anyone please help with this web.config error in an 5.1.422.256 installation?

Parser Error Message: Attribute type contains invalid type EPiServer.Implementation.DefaultBaseLibraryFactory, EPiServer.Implementation, becauseCould not load file or assembly 'EPiServer.Implementation' or one of its dependencies. The system cannot find the file specified.

 Line 130:      <add type="EPiServer.Implementation.DefaultBaseLibraryFactory, EPiServer.Implementation" id="ContentChannelFactory">

 /Andreas