Not all of OpenACS is Postgres 7.3 compliant yet. OpenACS 5 is compliant and the core packages in OpenACS 4.6 are compliant, but some non-core packages haven’t been fixed yet. Noncompliant packages will complain:

[24/Jun/2003:15:21:36][26623.196621][-conn:kurup::4] Error:  Ns_PgExec: result status: 7 message: ERROR:  Function  pa_collection__new("unknown", "unknown", "unknown", timestamp with time zone,  "unknown", "unknown", "unknown") does not exist
Unable to identify a function that satisfies the given argument types
You may need to add explicit typecasts

It is looking for a function with timestamp with time zone as one of its parameters, but most OpenACS pl/pgsql functions were defined as accepting timestamp. In PG 7.3, timestamp means ‘timestamp without timezone’. timestamptz means ‘timestamp with timezone’. So, the quick fix, is to change timestamp to timestamptz.

Read more at Bart’s posts in this thread.