Cannot use databases at /


I would like to set up a shared database, however I run into problems. My firewall is set to allow JabRef and my databases are both online (however, I did not do any extra configuration - is that necessary?). My error messages are different for each host.
these are the error messages for
JabRef 4.0-dev–snapshot–2017-07-30–master–81e6e8ed6
Windows 10 10.0 amd64
Java 1.8.0_144:
(I tried the 3.8 stable built which basically gave me the same results, except that it was a bit buggier: I only got the “Connection refused” error once for Postgres, and the communication link failure error in other attempts)