cache lookup failed for function 0

Hi,

select version();
PostgreSQL 15.4 on x86_64-mageia-linux-gnu,
compiled by gcc (Mageia 12.3.0-3.mga9) 12.3.0, 64-bit

As a test, rather than use INSERT, I recently wrote a python test script
to import some 8M & 33M record files with COPY instead. These worked with
last weekend's data dump. Next, I wanted to look into importing a subset
of columns using the below logic; but I'm getting "ERROR: cache lookup
failed for function 0". Re-running the same full imports that worked
Saturday, I now get the same error.

Could something in the DB cause this "function" error?

Simplified statements; just trying to import a subset of columns:

DROP TABLE IF EXISTS t;

CREATE TABLE IF NOT EXISTS t (
f1 text, f2 text, f3 text, f4 text, f5 text );

COPY t ( -- import only a subset of columns
f1, f3, f5 ) FROM '/tmp/foo.txt'
WITH (FORMAT CSV, HEADER, DELIMITER ' ', ENCODING 'ISO-8859-1') ;

ERROR: cache lookup failed for function 0
Where: COPY t, line 1
1 statement failed.

"function"? Is this referring to an implied/internal function? Searching
has not provided any clue, yet...

There are no user functions in the database:
ostgres=# \df
List of functions
Schema | Name | Result data type | Argument data types | Type
--------+------+------------------+---------------------+------
(0 rows)

It feels like something changed since the previously working script no
longer works...

Clues?

Thanks,
Pierre