If a pipe part file is located at
<installed_location>\design library\routing\piping\pipes\pipe.sldprt
and the Routing Library path is <install_dir>\design library\routing, then when you create a pipe, the short extension to the path, piping\pipes\pipe.sldprt, appears in the Route Properties PropertyManager.
Later, if you edit the route properties, the software tries to find pipe.sldprt at <Routing Library path> + <short extension>.
If you change the Routing Library path to, for example, <installed_location>\design library, (eliminating the \routing level), then <Routing Library path> + <short extension> no longer describes the correct path to pipe.sldprt, and the software cannot find the part.
It is important that all users who edit a particular route specify the same Routing Library path. Do not change this path after creating the route.