Push to origin failed

[Johannes Schneider <mailings@cedarsoft.com>]

Hi,

my Hudson isn't able to push master back to origin under some special 
circumstances:

The git-push command failed.
Command output:
To ssh://git.cedarsoft.com/git/com.cedarsoft.open
  ! [rejected]        master -> master (non-fast forward)
error: failed to push some refs to 
'ssh://git.cedarsoft.com/git/com.cedarsoft.open'


When I delete the .git directory and let Hudson do the same thing again, 
everything works well.
Of course I have verified the obvious first:


git push
To ssh://git.cedarsoft.com/git/com.cedarsoft.open
  ! [rejected]        master -> master (non-fast forward)
error: failed to push some refs to 
'ssh://git.cedarsoft.com/git/com.cedarsoft.open'

git log
commit 1e3d3d2c1a38cae75d60bbcd8bc739ed00597ad3
Author: Hudson <hudson@cedarsoft.com>
Date:   Tue Jan 19 15:25:10 2010 +0100

     [maven-release-plugin] prepare release open-2.0.6

commit 9b8b395a4f9f725aa82be61474835fd1f7d009a7
Author: Johannes Schneider <js@cedarsoft.com>
Date:   Tue Jan 19 12:00:14 2010 +0100

     license-plugin: added excludes



On my local computer, I have called "git pull" on master and then tried 
git log with that result:


git log
commit 9b8b395a4f9f725aa82be61474835fd1f7d009a7
Author: Johannes Schneider <js@cedarsoft.com>
Date:   Tue Jan 19 12:00:14 2010 +0100

     license-plugin: added excludes



So I don't see any reason why the push failed. Any ideas? How can I 
investigate this issue further?
Obviously I don't know enough about Git, so any hints are welcome...


Thanks,

Johannes