Fixed in jenkinsci/timestamper-plugin#47.
I have not yet released this fix, but an incremental build is available with the above changes which I believe should resolve this bug. Daniel Kirkdorffer, Rickard Dybeck, and T. Houdayer, are any of you interested in testing the incremental build before I release it?
Basil Crow I don't have a ready setup to test it against. Apologies.
Released in Timestamper 1.11.1.