When a build is run on a slave machine, the TestFlight plugin fails to upload the file to TestFlight.

java.io.FileNotFoundException: /Jenkins/workspace/Project Folder/build/Ad Hoc-iphoneos/Project_Name-Ad_Hoc-1.0.ipa (No such file or directory)

That is the correct path on the slave.

TestflightRecorder.java is using java.io.File in the perform method on line 114.
Using hudson.FilePath(FilePath base, String rel) with base = build.getWorkspace() will help to get the actual file.

Using FilePath.act can help if you need the code to be executed on the node where the data is located (in this case the slave).

http://javadoc.jenkins-ci.org/index.html?hudson/FilePath.html