Ahhhhh... I found the problem.
It was the "#" in the "/Show#" part of the final path - which needed to be URL encoded in order to work.
i.e... I made this change to the "MULTIshowSelectedSeg" calculation field:
Substitute ( GetValue ( MULTIfileTrackList ; MULTIshowSelectedValue ) ; "#" ; GetAsURLEncoded ( "#" ) )
This was to original, that was not working:
GetValue ( MULTIfileTrackList ; MULTIshowSelectedValue )