After an update to the new copy artifacts version 1.26 existing configurations are broken because all job names to copy artifacts from are empty. The configuration files still contain the correct names, e.g.:

<hudson.plugins.copyartifact.CopyArtifact plugin="copyartifact@1.25">
<projectName>rsc-trunk/label=$label</projectName>
<filter>*/.tar.gz</filter>
<target>upstream</target>
<selector class="hudson.plugins.copyartifact.StatusBuildSelector"/>
<flatten>true</flatten>
</hudson.plugins.copyartifact.CopyArtifact>

As you can see, most of our jobs use variables in the projectName tag. Maybe this is the reason for the problem. I actually cannot find a job without this, so I cannot verify this idea.

With 1.25 everything is ok.