A typo in our source code broke all pipelines that stash/unstash the built workspace. I figured out that it's because of a class file containing a non-ASCII letter (a dot-less "i" in the filename "Provıder.class").
Two pipelines to reproduce (output attached):
→ Fails to stash from slave.
→ Stash on master works, fails to unstash on slave.
I'm not sure if remoting is involved, but it stashes and unstashes fine on the master node, however that could also be due to implementation details for the master node.