This happened to me with an upgrade from WFA 4.2 to WFA 5.0 . There is a Perl script in ...\Program Files\NetApp\WFA\bin called InsertAdminUser.pl which is used by the install wizard to setup the admin user.

If any kind of Perl is already installed, try going to cmd, navigate to the above path and run with the following parameters:

perl InsertAdminUser.pl <admin_user_name> <admin_password>

Just to be on the safe side, you can also restart the service. It should reset the admin user, and I was able to login to the UI.