I am not able to add new module to the page. When clicking on Modules >> Add Modules menu, the cursor spin. Hit F12 and go to Console tab, it has an error below:

Failed to load resource: the server responded with a status of 404 (Not Found)

http://localhost/dnn742/DesktopModules/internalservices/API/controlbar/ToggleUserMode

This appears to be related to 2Sex content module after adding the App to the page.

How to troubleshoot this? There is no folder internalservices under folder DestopModules. Does it has anything to do with WebAPI or MVC?

Removed the 2Sexy module yet the problem is still there. So far, fixing the problem will require reinstall DNN.