The cleanup report "Authors with Author Data and One Non-Latin Title" errors out if an author has no language defined. The exact sequence of event is unclear, but presumably involves a disappearance of the author's language code between the time the nightly process runs and the time the report is displayed. "Make Pseudonym" is suspected as possible culprit.
Anonymous
Fixed in edit/cleanup_report.py 1.53, installed in r2016-147. Closing. (The cause of an author record losing its language code is still unknown.)