(1) If I logged in as admin, then then pretended to be a student and tried to "email the instructor", then I got the error message.
(2) I created a new student account by hand, logged in fresh as the student, and then "email the instructor" -- no error.
What's strange is that after I clicked the "email" button, webwork showed me a different screen depending on whether I'm in (1) or (2) --- I've sent along a screenshot for (1), whereas for (2) I do not have the "From" line. (there seems to be no way to attach two files)
For the record our system runs off the 2.13 liveDVD on a virtual machine, with kernel 4.10.0-32-generic. ;I'd be happy to provide additional info. Just let me know.