Postgres "database does not exist" log

Something is trying to connect to your cluster, expecting ABCD to be available.

My bet is someone saved a misconfigured connection profile in a database client like DBeaver, DataGrip, pgAdmin and it's just checking it from time to time, or that somewhere in your codebase someone left an example line without modifying the dbname that ends up in a connection string. There could also be a test case or rarely used code path doing something else and simply ignoring the failed connection attempt.

_{demo at db<>fiddle}

select pg_rotate_logfile();
create extension dblink;
select dblink_connect('','dbname=ABCD');

ERROR:  could not establish connection
DETAIL:  connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: 
FATAL:  database "ABCD" does not exist

create function readlog()returns text return
 pg_read_file('log/'||(array(select (pg_ls_logdir()).name order by 1 desc))[1]);
select readlog();

Ask other users of the database about dead connection profiles and grep your repo for this ABCD.

A FATAL error in PostgreSQL is an error that terminates a database connection.

That is your clue: such an error message is logged when somebody tries to connect to a non-existing database. After the successful authentication, PostgreSQL cannot connect the session to the database, raises the error you observe and terminates the connection.

Usually, that is a harmless error that you can ignore. If it happens too often, search for automated tools that are trying to connect to that non-existing database.