Hello I have an issue that is described almost the same as this issue: JENKINS-32512 that was closed.

The issue is as follows:

Create a job in a folder. Call it testfolder/sourcebuild.
Have this folder save an artifact.

Case #1 - Works
now create a job at the root, not in a folder. Call it getArt.
Use the copyartifacts plugin.
set the project name to testfolder/sourcebuild
set the job# accordingly.
Run job, works

Case#2 - Does not work
edit getArt
Add a string variable bad - set it to testfolder/sourcebuild
get the job# accordingly
set the projectname to $bad or ${bad}.
Run job. Cannot find source project.

I also tried adding it as /testfolder/sourcebuild.