Try using the url http://localhost:8080/portal/portal/admin. You should get the login box, login as admin,, after which you will be placed on the admin page. Then you can change the page name back to 'default'.
Oh, and if you really want to change the home page name to 'Welcome', you can edit the portal properties for the 'default' portal - change the 'Default child name' to 'Welcome'.
Thanks very much for your answer! it was great! I got it working.
All the best!