Update: sending a test post via an external application (Raven2Go) DID work. But I can't access the backoffice from there, so I cannot change my password or any settings. This is making me mad. :(

Are you sure that you've changed the password in the right database? This method can't fail. You must be editing another database or a table with different prefix.

I am sure. Plus, I only changed the password for one user, but NONE of my user/password combinations works. And the emails for resetting the password NEVER get sent.

Also, given that I'm able to post from the external application, I already know the password has been changed successfully. But I can't login.

well, that's a different story then.

Enable debug in /conf/_advanced.php and post here the part where it lists session/login info.

Sorry, I had posted the whole chunk without realizing what you wanted. I think you want this:

request:
vars: vars: Getting ReqURI from REQUEST_URI
vars: $ReqHost: http://www.lalaurette.com.ar
vars: $ReqURI: /blog/htsrv/login.php
vars: $ReqPath: /blog/htsrv/login.php
Hit: detect_referer(): self referer (://www.lalaurette.com.ar)
Hit: IP: 190.50.35.224
Hit: UserAgent: Opera/9.80 (Windows NT 5.1; U; en) Presto/2.10.289 Version/12.02
Hit: Referer: 'http://www.lalaurette.com.ar/blog/admin.php?ctrl=items&action=new&blog=1'; type=self
Hit: Remote Host:
Session: cookie_domain=.lalaurette.com.ar
Session: cookie_path=/blog/
Session: Session ID received from cookie: 2899139
Session: Session ID is valid.
Session: Session user_ID: NULL
Session: Session data loaded.
Login: login: NULL
Login: pass: empty
Login: User is trying to log in.
Hash key: 40
Hit: Recording the hit.
Hit:detect_useragent(): Agent name: opera
Hit:detect_useragent(): Agent platform: win
Session: Session data saved!

(And no, it's not a browser issue. Chrome does the same thing, as does Firefox.) :-/

This is suspect, right?

Query #8: Get User login

SELECT *
FROM evo_users
WHERE user_login = ''

I mean, should that string be empty?