[Setup] Cannot log into sites as the zmi administrator

[Lucia Colombo]

Hi all
I have migrated my Plone instance from Plone 2.1.4 to 2.5 (windows 
platform).
Migrating sites works, but if I try to log on to a site as the zmi 
administrator the "admin" user is not recognized.
This happens also for a brand new site in the instance.
Has anybody experienced anything like this?
If I log to the zmi and then browse to the site the session is logged in as 
admin nd I have the proper rights in the site.

Thanks&bye
Lucia