[packman] mythbackend error

[packman at howorth.org.uk]

When I tried to start mythbackend using YaST / Services / System
Manager it failed; there were two possibly relevant bits in the output:

/usr/lib/systemd/system/mythbackend.service:11: Invalid user/group name
or numeric ID: Productivity/Multimedia/Video/Players

and

mythbackend.service - MythTV backend service
   Loaded: error (Reason: Exec format error)
   Active: inactive (dead)

By trial and error, without really understanding what I'm doing, I have
got mythbackend to start by editing 
/usr/lib/systemd/system/mythbackend.service

I commented out the line

#Group=Productivity/Multimedia/Video/Players

mythbackend now runs when restarted. I believe that changing the value
to the name of an existing group also works.

I don't know what the line means, since I can't find documentation for
the User= and Group= lines, but I suppose that this is a packaging
error.