Comment 8 for bug 1678308

I solved the problem, when I replaced inside of
  ~/.config/libfm/libfm.conf
the line
  terminal=lxsession-default-terminal %s
by
  terminal=lxterminal %s
.

Thank you very much Walter! Your questions did push me to dig much deeper and to solve the problem in the end.
  Inside of Unity I got the following meessage when starting 'pcmanfm .&' in a terminal:
  $ pcmanfm .&
  ** (pcmanfm:14802): WARNING **: Couldn't register with accessibility bus: Did
  not receive a reply. Possible causes include: the remote application did not
  send a reply, the message bus security policy blocked the reply, the reply
  timeout expired, or the network connection was broken.

  ** (pcmanfm:14802): WARNING **: terminal lxsession-default-terminal isn't known,
  consider report it to LibFM developers
  $
This was the hint needed to solve the issue.
What I do not understand: Why did it work in 'lubuntu'?