UPDATE: Serendipity 1.5.2 release
Some SQLite upgrades from Serendipity prior to 1.5 to the current version might create a problem, that the database update cannot properly update the serendipity_authors database table structure to insert the new "hashtype" (int) column. Due to that column missing, this can lead to the inability to log in.
Sadly SQLite does not easily allow to ALTER a table structure, so unless you have a SQLite admin tool to manually that column, and until the Serendipity Team can provide a proper fix, you should not yet update. Due to Christmas 2009.1 getting released soon, this might takes us a few more extra days.
If you have NOT yet uploaded the new serendipity release and executed the update, you can simply copy the file sql/db_update_1.5-alpha1_1.5-alpha2_sqlite.sql onto your installation, which fixes the upgrade procedure.
Side information: Hashtype defines whether md5 (value '0') or salted sha1 (value '1') is used for the stored encrypted password of the serendipity_authors table. Once you login with a hashtype=0, serendipity will automatically convert your password to sha1, and adjust the hashtype column. This is why once we fix this issue in 1.5.2, we must make sure that no manuall updates you might have used are overwritten, that could lead to Serendipity no longer knowing which hashtype was used for your column. That requires some thoughtful thinking, and a bit more time instead of a hotfix that might make the situation worse.
Note that this ONLY applies to users UPGRADING and using SQLite. A fresh installation as well as using other Database types is not affected.