Hi Tjark,
The issue is with the client to have a updated object for the visiblity condition to work. So,
This should do the trick.
Don’t really get the sentence “ request object with several locations” because you continue with “When the user selects a certain location over the reference selector” That implies a 1-* relation and thus a single location selected using an association.
If you want to show a text when a location is selected using a ref selector you can use the following expression to show it conditionally;
$CurrentObject/[TheAssociationYouUseToSelectThe_Location] != empty