Hi, this is what I have discovered and reported to Support.
This might help someone else:
After doing several rollback of the upgrade and trying again and after many hours spent trying to figure out and to match against an empty Project (newly created) I could find that the upgrade has a serious bug!
The upgrade from 9.2 to 10.0 (6443 or 6410) adds an invalid setting in the Configuration table (this a hard to find when you cannot log in to Sitefinity)!
It adds a new sections (with new role ids that don't exist in Sitefinity DB) in SecurityConfig:
The main problem is the applicationRoles with new role ids, this will prevent any one to access the site and to login no matter what.
After removing these parts from the XLM in the DB table sf_xml_config_items it started to work as expected