This happens because the new default siteaccess from the platform installed during the upgrade process is not yet on the database.

Steps to reproduce:

1. After upgrading successfully from an updated 5.4 to platform according to the doc, go to platform's admin (http://<your_hostname>/ez);

2. You'll see the login screen without any formatting.

How I solved it:

1. Go to platformui admin interface (showing without proper css on the login screen) by logging in;

2. Go to "Admin Panel -> Roles";

3. Enter "Anonymous" role;

4. On the policy:
Module | Function | Limitation
user | login | SiteAccess( 3547135736)
click on "Edit limitations"
and choose both siteaccesses (the default siteaccess on the platform installation "site", and the old one "ezdemo_site_admin" you 'should' have added, from the previous 5.4 installation, if you used one from the previous 5.4 installation)

5. After this delete browser cache and cookies and go back to login screen. It should now show correctly with proper css.

Perhaps some info referring this step (although not properly mandatory, since it only happens if one takes advantage on using the default used siteaccesses) could / should be added to the doc