I had this problem previously on another machine (under 14.04) but now I have it on a different machine under 16.04.1

Choosing shutdown from the top right hand corner menu does not work but "sudo shutdown -P" does.

Sorry for cross-posting, but this is getting annoying.

Note that I have both SpiderOak and Dropbox running on both machines, but quitting both applications before choosing shutdown from the menu does not appear to work.