Socket command type I unknown

Good evening,

I hope that I am posting this to the right place. If not, please
direct me to the appropriate mailing list and I will send to that one
instead.

Background Info:
Debian Linux (Sarge)
Server A -- Apache 2.0.54 + mod_perl + DBD::Pg
Server B -- PostgreSQL 7.3.4, Compiled, not debian package
(yes, I know we should upgrade)

Although I haven't been able to pay much attention to it until
recently, we occasionally get the following message in our apache log
file. I'm looking for information on what it means:

DBD::Pg::db selectrow_array failed: FATAL: Socket command type I
unknown
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
DBD::Pg::db selectrow_array failed: no connection to the server

After this, we get a series of errors like this:

DBD::Pg::db selectrow_array failed: no connection to the server
DBD::Pg::db selectrow_array failed: no connection to the server
DBD::Pg::db selectrow_array failed: no connection to the server

And I -know- that's causing trouble on my web server. :) A restart of
the web server 'corrects' the problem by reestablishing the
connections to the database. I suspect this might be caused by a
mismatch between the client libraries on Server A (7.4.X) whereas
Server B is a 7.3.X install. (I'm working on correcting this ASAP,
which will also get us on 7.4.)

Basically my question is this: What does this error indicate? I can't
seem to find much about it on the net and to be honest, I have become
rather illiterate in C over the past several years, so reading the
source is not really a viable option.

Any info would be appreciated. Thank you for your time.

--Joel