This is caused by a breaking change in Episerver 11.0.0 where some objects where moved from the base EPiServer.Framework module to the EPiServer.Framework.aspnet module. To fix this go into your web.config file and look for:
<add virtualPath="~/cms/" physicalPath="modules\_protected" name="ProtectedAddons" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider, EPiServer.Framework"/>
And update it to read:
<add virtualPath="~/cms/" physicalPath="modules\_protected" name="ProtectedAddons" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider, EPiServer.Framework.aspnet"/>