User: root
In the current release of Leap 15.1 thunar cannot communicate with xfdesktop.
So you can create a starter on the desktop, but you cannot launch the
application. You get the message "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."

The version of thunar (1.8.4) is incompatible to xfdesktop (version 4.12.4).
See the discussion: https://forum.xfce.org/viewtopic.php?id=12890
As discussed there the issue is only for root user.

Workaround is to install thunar 1.6.14 from Leap 15.0. 
Solution see post #9: upgrade xfdesktop to 4.13.x.

I have no tumbleweed, but the combination should be better: xfdesktop 4.14.1
and thunar 1.8.9.