psql (client rpm in 9.5) is not consistent with build default of 9.5 server rpm. Is that a possibility????
I suppose - but to verify it looks like your connection syntax is not really compatible with the socket style, docs say you should pass socket path with to the -h or --host - maybe pass your socket in as:
h hostname --host=hostname
Specifies the host name of the machine on which the server is running. If the value begins with a slash, it is used as the directory for the Unix-domain socket.
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-- Robert W. Burgholzer 'Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity.' - Charles Mingus
