Christian Wenz points out a "gotcha" for those upgrading Serendipity to the latest 1.5.x version - an issue with a SQL script not being run.
I just updated Serendipity to version 1.5.1 on one of our servers; yet afterwards I could not log in anymore. Also, Serendipity reported that version 1.5.1 was present, although I did not run the update script from the admin console yet. At first I thought I did something wrong, but a s9y forum posting described a similar issue.
The issue came from a SQL update script that hadn't been run when the upgrade process thought it had. He includes the two SQL statements you'll need to run to fix the problem.