I recently upgraded from 16.04 to 18.04, and I found that I couldn't view any of the other login-able users.
Unlike this question: Cannot select file for user profile image in 18.04, there just isn't any users to edit.
They exist when I check /etc/passwd/
I don't have a "for sure" answer, but I can tell you this.
I'm 90% certain this was due to invalid data from multiple Ubuntu upgrades that never got quite settled. Some config that powered this gui probably was using information that was incorrect. How to update that is a mystery beyond me.
When I re-installed Ubuntu 18.04, everything (obviously) worked fine.