After restarting Jenkins (using http://servername/jenkins/restart) Jenkins does not show any old builds. The workspace is empty ("A project won't have any workspace until at least one build is performed."). After manually reloading the data from disk the old builds show up again.

In the log I see exceptions:

hudson.util.IOException2: Unable to read /var/lib/jenkins/jobs/Myproject/builds/2012-02-14_01-32-26/build.xml
Caused by: com.thoughtworks.xstream.converters.ConversionException: Cannot deserialize object with new readObject()/writeObject() methods

I've attached the starting up logfile.