last_login is not an IMP preference. You probably upgraded from an IMP 
3 version and didn't follow the upgrade instructions.

Background:
I'm using the Horde-Framwork with imp-Authentification.
The preferencies backend is LDAP.

When I'm including ingo in imp then the last_login preference is not stored!

Reason:
- The imp-preferencies are loaded (old value for last_login is retrieved)
- Auth.php sets the new value for last_login
- Then the ingo-preferencies are loaded and overwrite the new value 
for last_login

Resolution:
Store the new "dirty-value" for last_login after setting it:

Code snippet:
      // Set the user's last_login information.
      $last_login = array('time' => time(),
                                    'host' => 
@gethostbyaddr($_SERVER['REMOTE_ADDR']));
      $GLOBALS['prefs']->setValue('last_login', serialize($last_login));
      //NEW: ************************Store it ! *********************
      $GLOBALS['prefs']->store();