We upgraded from 4.6.2 to 4.9 recently. All is working well, it seems, except for the login / authentication module. When attempting to login, no username/password box appears and instead we are greeted with a module load warning:

Module Load Warning
One or more of the modules on this page did not load. This may be temporary. Please refresh the page (click F5 in most browsers). If the problem persists, please let the Site Administrator know.

Please suggest on this.

Well... what I might suggest is that you ask the friendly folks at DotNetNuke since it is their application package and you've provided no code for anyone to work with in your post.

Generally speaking if you're having a problem with a pre-packaged software solution the best people to ask would be the people who pre-packaged it :)

Just my opinion though.

Many reasons can lead you to this condition such as module not found, Usercontrol not compatible or contains error etc. You need to login as super user and view the exact reason for this.