Did the same with webforms. Same error
Added this and it works
I think your solution is right to the point.
Root error is show in the log:
[InvalidCastException: Unable to cast object of type 'System.Web.Security.RolePrincipal' to type 'EPiServer.Security.VirtualRolePrincipal'.] EPiServer.Labs.LanguageManager.Controllers.LanguageStore.IsCurrentUserInAdminRole()
I am setting up a demo site and having some issues with episerver langues add-on.
Installed new Episerver web site project "alloy mvc" from Visual studio. I run the site and enables one extra language and enables new language in edit mode.
When i add the languages gadget i get this error (and languages gadgets is emty). It is possible to translate pages but the gadget is not working.
I have also tried installing add-on from store. Also tried updating all episerver nugets.