Run the MAPLOGOUT command.  Does that make the login box appear?

If it made it appear make sure you don't check the "Log in this user next time without prompting" box.

If that doesn't work you can check to see if the users folder in the database has been deleted.

Old folder structure is located here:

...\PM Shared\Database\USERS

New folder structure is located here:

C:\Users\Public\Documents\Autodesk\Fabrication 2017\Imperial Content\V3.04\Database\USERS