When you are "logged out" but still have the session cookie, which HTTP does your Browser send to Serendipity (check a tool like LiveHTTPHeaders for Firefox)?
I cannot reproduce the Problem right now since I cleared my cookie cache :-( But I suspect it to happen again, as I was only trying to ...

As said, the php session is destroyed when i logout through the log out link, but after that i can enter the admin area without a new login.

And now i'm in the admin area without a valid php session, everything seems to work unless i try to modify data (like edit an useraccount, oder post an entry ...

I did a little testing and it seems to be a cookie/session Problem.

When I clear my cookie cache after login everything works fine. After Logout via the "Abmelden" link the php session is purged (from /var/lib/php5) but the browser cookie is still there. When I call the serendipity_admin.php I'm ...

I have the same Problem. Some people cannot post and see the admin area in the preview area.

serendipity sometimes seems to not write sessions.

I use Debian Sarge with an backported 5.2.0-8+etch3 from Debian Etch.
MySQL is 4.0.24-10sarge. Serendipity is 1.1.2.

According to the config session ...