When clicking Sign In for the NBS Plugin within Revit a spinning wheel appears but no sign in box appears. Clicking the support Icon again the option for Sign In is still available. This indicates that the Remember Me checkbox was selected and the users password has a special character in it (!"£$%^&*)
Solution
1) Sign in to TheNBS.com and select the My NBS button at the top of the screen
2) Click the Manage Account box And select Security
3) Click Change Password and reset your password.
4) Now, in the registy editor (start menu type Regedit) browse to Computer\HKEY_CURRENT_USER\Software\NBS\NBS Plugins and delete all stored logins
5) Relaunch Revit and you should now be able to sign in successfully.