May be fixed with https://github.com/jenkinsci/copyartifact-plugin/pull/14 .
I'll try that.

In my environment, the problem happened in a different way:

• Copy Artifact plugin 1.27:
  • Jenkins starts well.
  • Copy Artifact is displayed in Build Step of jobs.
  • When Copy Artifact runs, an error occurs and the build fails (This should be JENKINS-15977).
• Copy Artifact plugin 1.28:
  • An error occurs when Jenkins starts up. Failed to load Copy Artifact plugin.
  • Copy Artifact is not displayed in Build Step of jobs.

Additionally, when I packaged Copy Artifact plugin 1.28 in my local environment, the problem at the startup does not reproduce.
The root cause may be the version of maven-hpi-plugin to use in packaging.

I found this has nothing to do with JENKINS-15977.
Even after fixed JENKINS-15977, I reproduce this.

It seems depend on the version of Java compiler used in packaging.
Packaged with JDK 1.7, it is not reproduced.
Packaged with JDK 1.6, it is reproduced.

I attach the stacktrace when reproduced.
I tested with Jenkins 1.549.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/CopyArtifact.java
http://jenkins-ci.org/commit/copyartifact-plugin/f3a75a6bcaec0b600ddb1cf76ae1126f960c791f
Log:
[FIXED JENKINS-21274] Fixed startup failure when maven-plugin is disabled, compiled with JDK 1.6. SEE https://wiki.jenkins-ci.org/display/JENKINS/Tips+for+optional+dependencies for details.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/CopyArtifact.java
http://jenkins-ci.org/commit/copyartifact-plugin/ddb5694d400987e0af0e879ac525c4ab759e69ee
Log:
Merge pull request #30 from ikedam/feature/JENKINS-21274_UnexpectedClassLoading

JENKINS-21274 Fixed startup failure without maven-plugin when compiled with JDK 1.6

Compare: https://github.com/jenkinsci/copyartifact-plugin/compare/d00c85521648...ddb5694d4009

This should be fixed in Copyartifact 1.30 (will be available in a day). Please try.