JabRef 5.1 Linux portable

What is the proper method to run JabRef 5.1 Linux portable?
A) I tried to run JabRef from “/JabRef/bin” and get “Unkown file type” with 5.1, this worked with JabRef 5.0
B ) I also tried to run it from “JabRef/lib/runtime/bin” and double-clicked on the JabRef file, this works.
What is the difference between starting JabRef using the path as in A or B? Or is “/JabRef/bin” possibly broken? I am running Linux Mint (Tricia).