New try using Firefox, after central login with my main user name Alex brollo.
As I click on the left button "Login or Register Mediawiki", at https://phabricator.wikimedia.org/auth/start/?next=%2FT209939, an OAuth popup opens; I click on "Allow" button; a page "Create new account" opens, user name "Alex_brollo", when I try to fill required fields (email) and click "Register" the page tells that both user name and email are duplicated.
Then I tried to ask a new password for LDAP user Alex_brollo and I got a mail from Wikitech, with a temporary password for user "Alex brollo" (no underscore!):
"The following user account is associated with this email address:
Username: Alex brollo
Temporary password: " (follows a password)
I tried LDAP login with that username (both with space and with underscore) and the new password: both username and password are "Invalid".
I'm sure that previusly I contrbuted to Phabricator by mediawiki login with no problem; presently I'm so confused (and frustrated!) that I can't remember if I have, or I have not, a LDAP account.