For SSH sessions I'd like to add that the error still persists here. Nastyly it's happening for me on a laptop with broken display that I use as a "server" (no graphical login). So I'm -- among others -- not able to add users or change time.
While reading here I couldn't find a solution that would let me use users-admin without a display, i.e. graphilcal login?
Just now I noticed this message while fireing up users-admin sudoed:
** (users-admin:26835): CRITICAL **: Unable to lookup session information for process '26835'