Hi, I think you should add a note to the installation tutorial for RoundCube that when using Postgresql as the database, you must enable TCP/IP connections. Postgresql uses Unix domain sockets by default and many other web apps that use Postgres communicate with it in that way rather than through TCP/IP, so users may not be aware that they have to enable it for RoundCube. The way they do that is to add the option -i to postmaster when starting, or:

-o '-i'

to pg_ctl. On Mac OS X, I had to modify the default Postgresql startup script to include this option.