We're using the latest versions of Arcanist and Phabricator (as of 7am PST, 18 April 2015). One of my employees (and one of the few using Windows 8) tried twice to create a differential via arc diff. The differential was created successfully, with all of the files and folders, but the files themselves were confirmed blank. (They both showed up as blank on the Diff, and were blank when downlaoded via arc patch.)
She has previously been able to successfully arc diff on the same system, though she hasn't done one in a few weeks, so I don't know exactly when in the Phabricator updates this broke.
The Linux version of Arcanist is working perfectly. I was able to upload her files to the diff for her from my end (Ubuntu 14.10).