During setting up a first project, I noticed, that there is somethig wrong with the subverion credentials. In a first step I gave credentials for my own account: kai.hofmann. In a second step I changed these to a special jenkinssvn user. But within the projects workspace there are still the kai.hofmann credentials. Changing again the credentials via the projects configuration back to kai.hofmann - the jenkinssvn credentials appear in the workspace directory!
Giving the same credentials two times works as a workaround for this bug.