I suppose this have started to occur after the upgrade to sitefinity 6.1. In sitefinity 6.1 the MVC version used in sitefinity was upgraded to using MVC version 4.0 (before it was MVC v 3.0).
The assembly reference for System.Web.MVC must still be pointing to System.Web.MVC version 3.0. Please check the assembly references for the site for System.Web.Mvc and make sure it is pointing to version 4.0 (here
is a screenshot).
After this compile the project and both registering and using MVC widgets will be possible.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items