Here is my use cas :

Redmine side :

• admin / configuration / authentication / [x] activate api rest
• my account / show api key / copy api key

Jenkins side :
1- Create a minimal project (free-style)
2- Set up Redmine such as :

• redmine webiste : http://192.168.2.1
• redmine project name : myprojname
• redmine versin 1.0.1
• Project is located at http://192.168.2.1/redmine/projects/myprojname
  3- Add post build action :
• Paste API Key
• Save
  4- Launch build

And then I get the following trace :

Démarré par l'utilisateur olivier
Building in workspace /home/user/.jenkins/jobs/Agreger Redmine/workspace
FATAL: NotFoundException received, which should never happen in this request
com.taskadapter.redmineapi.RedmineInternalError: NotFoundException received, which should never happen in this request
at com.taskadapter.redmineapi.RedmineManager.getProjects(RedmineManager.java:156)
at hudson.plugins.redmine.RedmineMetricsCalculator.getProject(RedmineMetricsCalculator.java:99)
at hudson.plugins.redmine.RedmineMetricsCalculator.calc(RedmineMetricsCalculator.java:46)
at hudson.plugins.redmine.RedmineMetricsPublisher.perform(RedmineMetricsPublisher.java:54)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:776)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:726)
at hudson.model.Run.execute(Run.java:1601)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:241)

I Googled and couldn't find any help about this trace. I tried to add this post build action to a real built project, but the error is the same.