unsupported frontend protocol

I've painted myself into a little corner here:

I pg_dumped a 7.4.3 database, created a database of the same name on a
7.3.4 server, psql'd into the new database, and \i'd the dump file.

The database was created although there were a variety of errors which I
realized were due to 7.4.3 and 7.3.4 SQL incompatibilities. I had also
added a new entry to pg_hba.conf. After restarting the 7.3.4 postmaster
I started getting an endless series of "FATAL: unsupported frontend
protocol" messages, even when not using psql. The only other access to
the cluster is by PHP 4.2.2, and nothing accesses the newly restored
database.

Since the db had errors anyway, I restored the old pg_hba.conf file,
dropped the new database, and restarted the postmaster -- i.e.
attempting to wipe out all changes. HOWEVER, the protocol error message
persists, endlessly and infuriatingly! Searching on the error message
text doesn't return anything enlightening.

Can anyone suggest why this is happening or where I find a log with
further details. Thank you in advance

John Gunther