Cannot Connect To Db From Local Server

We have a legacy PostgresSQL 7.3.4 db on Solaris which has recently (as in last 24 hours or so) begun refusing TCP/IP (JDBC) connections from the same server the db is on.
The db is up and running-psql connects; I can access the db via pgAdmin III from a PC; I can connect via JDBC from a PC.
However: any JDBC call (whether command-line app or web app) running on the db server fails with this message:
org.postgresql.util.PSQLException: Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
And the postmaster log contains the following message:
FATAL: unsupported frontend protocol
Mind you... I am using the same JDBC jar (pg74.216.jdbc2.jar) and connection URL in code on my PC and the Solaris server.
Our pg_hba.conf file contains the following lines:
local all all password
host all all 127.0.0.1 255.255.255.255 password
host all all 0.0.0.0 0.0.0.0 password
And "tcpip_socket = true" is enabled in postgresql.conf.

Any ideas?

David Rickard
UCLA Library Information Technology
drickard1967(at)library(dot)ucla(dot)edu
310.206.9780