Just migrated to pressflow and now users can't log in.. any ideas? I just migrated to Pressflow (newly downloaded) from vanilla Drupal install, and all of a sudden users cannot log in.  The page simply reloads, but there is no authenticated user.

I am getting the following errors when I flush the cache (and I got them after running update.php):


  Warning: Attempt to assign property of non-object in
  drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). Warning: Attempt to
  assign property of non-object in drupal_write_record() (line 3488 of
  /home/traders/public_html/includes/common.inc). User warning:
  Duplicate entry &#039;0&#039; for key &#039;tmd&#039; query: INSERT
  INTO blocks () VALUES () in _db_query() (line 147 of
  /home/traders/public_html/includes/database.mysqli.inc).


Any help would be so so greatly appreciated!  If I can't figure this out in next couple of minutes, I'll roll back to normal Drupal.

Thank you!

**Edit:  My admin account was already logged in after I ran update.php and is working fine.  It seems like a problem creating a new session.

MORE INFO:  When a user logs in, my log says "Session opened for XXXXX." but it's quite clear the account isn't logged in ($user->uid is not set)
Not sure if that's going to be of any help for you, but I had kind of similar problem - on staging site, after upgrade from Pressflow 6.16 to 6.26, users were not able to log in anymore - or actually, according to Watchdog, they were logged in, but still being redirected to log in form.

No warning like yours though, although I assume that in your case they we caused by your admin account already logged in.

At this time prod site was still running Pressflow 6.16.

What helped in my case was changing $cookie_domain value in settings.php file on the staging server, which "unlinked" staging and prod cookies. 

Essentially, before both staging and prod sites were using the same $cookie_domain (let's say it was mydomain.org), and everything started to work fine after changing it to stage.mydomain.org on the staging server.

Perhaps the problem is with the different way of handling cookies, and once our Pressflow is updated on prod server too it will again work fine with the same $cookie_domain, but this is still to be tested and confirmed.